Definitely do the plasticine (bump) clearance check, definitely determine true TDC by the positive stop method. If you are anal about this stuff like me, you will want to measure that the head has been cut squarely, measure that the block has been cut squarely, measure that the pistons sit square in the bores...and measure the ring end gaps of all piston rings.
Calculating your actual CR isn't too hard, just requires some careful measuring, that's all.
Get yourself a small sheet of plexiglass (perspex) that will fit over the chamber / bore. Buy yourself some graduated syringes from a pharmacy, vet supplies etc ...they are pretty cheap... a selection (say 10ml, 5ml and 2.5ml) with graduated markings on the side makes it quicker to do.
Position the plexiglass over the chamber, use vaseline / petroleum jelly smeared around it to help it seal up. get the head so it sits pretty level and mark a couple of points on the plexiglass that fall within the chamber, one is going to be an inlet hole, the other will be your air vent.
Drill the holes out... maybe 2 to 3mm is sufficient...put a bevel on the inlet hole which will help you not spill any fluid and get the tip of the syringe in... put the plexiglass back on. Have a spark plug screwed in and the valves fitted, and both valves closed (if the cam is fitted )
Carefully fill the space in the chamber with fluid, very light oil (cooking oil works ok) kerosene, a mix of both these to get a nice not too viscous and not too runny liquid. Keep note of exactly how much fluid it takes to fill the chamber, do it a couple of times so you know your results are correct by repeatability. For total accuracy do all four chambers but that's not 100% necessary. This is your true chamber volume.
Do the same thing to the flycuts on the pistons...measure their true volume.
At true TDC, determine exactly how much positive deck your pistons have in your block using a good flat edge and feeler gauges....do this to pistons 1 and 4 at least... all four if you can be bothered.
I have another set of exactly the same pistons your using so I can determine the piston ring land volume and piston head diameter for you.
Measure the head gasket thickness of the head gasket you have ... and note the brand and type you have...as we will need to do a little educated guesswork about it's crushed thickness
post all the numbers here and I'll help you do the math....it's actually pretty simple once your shown how.
SteveC