Priming a big beer after lagering

[bjhbrew]

I’m hoping someone can provide some advice with the priming and bottling of a Belgian quad I brewed about 7 weeks ago. The beer is the westvleteren 12 clone by csi. Brew day went well and after 10 days in primary the beer had attenuated down to the expected 1.012 with a temperature ramp up to about 80 degrees. At this point I transferred the beer into a keg so that I could purge it with co2 before lagering at 50 degrees for a month as per the recipe. Now here’s the part where I need help... I kept the co2 hooked up during the temperature drop to keep a positive pressure in the keg at about 5 psi. Then when I reached the lagering temp I hit the keg with 20psi before removing the co2 tank just to make sure the lid would stay sealed. So now I want to bulk prime in the keg and bottle from the keg with a bottle filler (not force carbing this one) but I’m wondering if the beer will already have some carbonation from my use of co2 and if there is a way to figure out how much priming sugar to use. Also wondering if I’ll need to add yeast at this point (after a month at 50 degrees and highish abv). Well that’s all, thanks for any help!