I have recently retrofitted two lithonia 2x20 strips back to preheat. Is there a way I could achieve lead lag with a capacitor and starting compensator or am I missing something? I was talking with another member and they said I was going to kill my ballasts if I did that. I believe I saw a member here a while back make a lead lag circuit with a couple of chokes and a cap. Regards, Solanaceae.

Assume you mean sing just series chokes for the ballast, so plain one for the lag section and with series capacitor for the lead branch...
If the capacitor is calculated right, there is no risk for the ballast choke: The capacitor's reactance should be double of the choke reactance (just the inductive part counts here).
But very frequently is problem to determine the choke reactance: The series chokes tend to supply the lamps by different currents (there is no way single choke could supply both the 15 and 20W lamps by the rated current) and the capacitor should be based on the ballast.
One way is to start with a bank of capacitors: Starting with a value corresponding to the ballast rated current (assume perfectly lossless ballast):

C = Iballast /(2*2*pi*60*sqrt(120^2-60^2)). (for a 0.25A ballast it would be 3.2uF)

The rated voltage should be at least double the mains, so at least 250VAC (for the use with starting compensator I would recomment even a 350VAC type, to prevent it from being overstressed during the preheat).
Because of the ballast resistance, you will get lower current than the rated Iballast, so then increase the capacitance a bit to reach the Iballast. Never go above the rated Iballast, staying a bit below is not much a problem.

Then the other consequence is, the lead section tend to maintain that current better over changing the load voltage, so it won't increase the current for the preheat. That means either the lead section would be more "blink happy" and wear a bit faster. For that you would need a starting compensator, but that would mean a special choke designed so, the current will be the required 0.3..0.4A. I doubt you will find a ready made compensator for this, the ones intended for the old F40T12 won't have correct inductance. But you may try one, you may try even another ballast choke of the same type, but don't use it, when the preheat current exceeds the 0.4A level.
It is not a bad idea to bypass the compensator by some ~150V VDR, it will allow the full voltage from the starter opening to become a starting pulse.

Thanks for the info. I'm using a .35a ballast. The question is how should I wire another choke in as a starting comp?

You should first check, what would be the preheating current without the compensation.
The problem is, with the second such ballast in series, the circuit becomes really a series LC tuned exactly on the 60Hz, so if there would be no losses, nor saturation, the currents and voltages would rise to infinity...
Of course, the losses, but mainly the core saturation does limit it somehow, but it could be way above what the components are designed for. Normally the starting compensator has lower inductance and saturates at a bit lower current than the main ballast, so the circuit remains tuned above the 60Hz and the compensator provides a means for rather soft current limiting (by detuning the circuit further without loosing all the inductance at once).

By the way in Europe is quite common to use preheat lead-lag combos without starting compensators in the lead leg and it works quite well, so I don't thing it would be so severe problem...

How do I measure the preheat current? Could I look up a spec sheet for the advance chokes?

Could I use the same value of cap on the f17t8 tubes as well as on the f20t12, or is it a completely different story with them?

How do I measure the preheat current? Could I look up a spec sheet for the advance chokes?

Measure: AC A-meter instead of the starter, shorting jump wires instead of the lamp filaments...

Target value (where to aim): Detailed lamp specification and/or standard governing that lamp type spec's. But both are hard to get...
General "rule of thumb" is 1.5x the nominal arc current (do not confuse with the line current printed on the ballast case; the line and lamp currents equal exclusively for not compensated series choke ballasts, but nothing else) for 1..2s preheat time.

Could I use the same value of cap on the f17t8 tubes as well as on the f20t12, or is it a completely different story with them?

The cap is "bound" to the ballast choke, not the lamp.
For each ballast model you have to pair specific capacitance value, the different ballasts even for the same lamps tend to differ quite a lot.

So if you will operate both lamps on the same type ballast (really the same model and make), it will work.
If you intend to use different ballast type, it won't (except the ballast happens to be really exactly the same inductance).

The ballasts match. I just need a 3.9 uf cap.