Let me wade in where angels fear to tread.
I cant put my hand on the source but my understanding was the pressure drop from the top of the radiator down the small diameter steel bleed line to the tank locations is about 2-3PSI. If the small tank is running a 15psi cap and the bleed line from the radiator is feed into the large tank you can have a 12-13 PSI cap on the large tank and the system will be pretty much in balance. I did this and included an inline valve in the radiator top bleed line such that once the system is pressurised , you can open the valve and you can hear any bleed into the large tank with no associated loss of fluid out of the large tank. There is almost no loss of fluid with this arrangement and the system sees the full small tank cap pressure.
Initially I did get caught by the depth of the necks on the vendor supplied replacement tanks. By recollection they were 26mm not 19mm like the old tanks which was a bit of a trap for a trusting young bloke from the bush. Just something to watch out for as 0 PSI doesn't work.
Having trouble with the patented fluid level measurement device- may have to buy some undies to get the elastic .....!