If it's always been OK and now it isn't then it won't be calibration that's the problem unless the ECU has forgotten calibration settings (by which point the ECU is more or less broken really), it will be due to a hardware problem with either the LPG system or the vehicle (lambdas usually).
You probably shouldn't be at the point of attempting to calibrate yet but If you do try to calibrate....
If it's Etagas it will take forever to calibrate unless you drive it to keep rpms within the certain ranges until all the range boxes are learned. It would be no good trying to hold rpms in the range boxes while not driving as there wouldn't be enough engine load and all the learning/autocal done would be wrong.
Since you say you have the ability to switch individual cylinders back to petrol it would seem you have a sequential system.
If you have a sequential system don't try to get ginj = pinj, do try to get pinj to remain close to unchanged when you switch between fuels.
It doesn't much matter on a P38 if ginj is 1.2 or 1.5 or even 0.8 x pinj at idle. The at idle multiplier should depend on spec of components fitted and pressure settings... If you have Matrix injectors I might expect multiplier of 1.1 at idle falling to 1 at flat out loads, if you have some of the worst performing Tartarini injectors fitted I might expect mutliplier of 1.2 at idle to 1 at high loads, these figures are only so close (1.1 / 1.2) because the P38 idles with 4ms pinj anyway (and 4 x 1.2 = 4.8 which is an easy minimum pulse duration for nearly any LPG injector). But really, knowing how half of P38s were/are setup by installers of the day I might expect anything between 0.7 and 1.7... but it should still work OK.
You should never see ginj remain the same multiple of pinj across the load range, ginj should become a lower multiple of pinj as load increases (at anything much above low loads at least) and for best results from any components you have fitted ginj should be close to pinj at high loads when at the same time mixture at high loads is correct. Or calibration is wrong, or you have a pressure sensor problem.
Popping from the exhaust could have various causes including implications from an overall too lean or too rich mixture or a dodgy ignition system, but if you only get popping from the exhaust when you switch a specific cylinder back to petrol it could be that your gas injector plugs are on the wrong gas injectors... and in that case they might even have been put on the wrong injectors intentionally by an installer who was trying to hardwire an 'advanced injection sequence' (on something as simple as a P38 this would usually have been because they'd fitted a setup with incorrect combination of nozzle size and pressure).
@Dhadara, what did you calibrate to .45? The best way to see how well an engine is running on a certain fuel is to check inlet manifold vacuum readings at the same time as ensuring lambda is correct, a happy engine will draw more vacuum, there shouldn't be any difference between petrol vacuum and LPG vacuum. You can get more vacuum with a rich mixture but remember we covered correct mixture with lambda and we're looking for most vacuum with correct mixture.