As far as I can tell, reinforced bulkheads originally had a mass that was 7.5% of the parent ship's hull, and military (and reactive/mirrored) bulkheads had 15%. In cases where this resulted in a decimal value (e.g. Adder), the mass was rounded to the nearest ton.
Of course, that was only for the ships that were available when the game launched 5 years ago, and only as those ships existed 5 years ago. Nowadays, most ships seem to deviate from this rule for one of 2 reasons:
- The ship received a reduction to hull mass, but the mass of its bulkheads did not change (e.g. Orca, Type-7, Type-9)
- The ship is design derivative of an existing ship, and therefore has the same bulkhead mass as the ship it was derived from (e.g. Viper IV, ApsS, Corvette)
The only exceptions to this I can find are the Alliance trio of ships and the Kraits. The bulkheads of the Alliance trio are based on the Chieftain's hull mass, but have a mass fraction of 10% and 19.5% for reinforced and military bulkheads, respectively. Similarly, the Krait bulkheads are based on the Krait II but have mass fractions of 11.25% and 21%. I have no idea why FDev decided to do this since these ships are largely considered to be heavily modified versions of the Federal trio and Python and the bulkhead masses from the existing ships would have worked perfectly fine. Additionally, if FDev decided to go this route for these ships, I'm not sure why they didn't also use different mass fractions for the Mamba (which came out after 4 of the 5 ships) and instead used the bulkhead masses from the FDL.
TL;DR: There was a rule, then old things got changed and new things got added, then the rule was selectively ignored for a while. It's a bit of a mess.