buoyancy of Marelux 180mm glass dome port

Hi all,

Working on counterweighting my setup- I am currently running with a sony A7CR and sony g 20-70 f4 in a marelux housing with the 50mm extension ring/ 180mm dome port. The dome is slightly positive and attempts to always flip upwards. In works with a savy 3D printing friend to build a weight collar on the extension ring to offset the current buoyancy. does anyone have experience with this or a similar system that could help me get a rough idea of how many grams I should begin testing with?

Was going to begin with fixing ankle weights to the rig, so anything more specific would be very helpful!

Cheers!