I've come upon a deal on a 509 BBC...Bowtie block, dimple rods and Ross Pistons(he says) that look to have .100 dome on them.
Wondering what compression I would end up with using those pistons and 100cc heads?
How about with 119cc heads?

I figured about 10.75 with the 100cc heads? Close?
Thanks ahead of time.

Damn near impossible to answer without measuring the effective volume of the dome.
It depends on dome design, valve notch depth,,,,,

Best to call Ross & give them the job number if you have it or describe the piston & get a guess.
If it is a shelf piston they should be able to tell you what you have.
I have a Ross catalog but I don't think they give dome height. I'll try to remember to look later today.
If I forget email me & rattle my cage if you don't get an answer ;)