I'm doing a lightweight build with a JP Ultra LMOS BCG, JP captured spring, and an SLR adjustable gas block.  Once this is all assembled, how do I go about tuning the gas system?  

I figured start with it mostly closed, then open it one click at a time, but how do I know when to stop?

When you load one round, and it locks the bolt open.

Then I'd go a click or two past that.

Also be aware that tuning is a bit dependent on the ammo you use.  If you will be shooting this gun in competition, you should tune it with the ammo you plan to shoot.  Going a click or two more open after you get the bolt to lock back, is for reliability when the gas path and chamber gets dirty.  Good cleaning regimen is a good idea.  Your buffer weight and spring strength will also play a role in your tuning.  Might want to have a buffer that you can play with weight and also a couple different buffer springs including a reduced power spring.  Good luck, and report back your tuning experience, please.

Edit: sorry, saw that you're using a JP captured buffer/spring.  I think JP makes a spring kit to let you tune it, but with a JP SCS and lightweight BCG, you have fewer variables to have to worry about.

I almost ran this exact setup but went with the JP lmos instead of the ultra. I'm starting with the JP scs lightest spring and the slr gas block one click open. I'll be shooting either tula or weak target ammo for the setup. If the bolt is slamming or not feeding the round I'll put in a heavier spring on the Jp scs. Once tuned I'll go one or two clicks more depending on the other variables and the gun will probably never see tula again.

I am a few months from completion though. In waiting on a tax stamp.