I can’t remember how CFD for gold/silver works. On a spreadbetting account, it’s a bet per point, so that’s easy to work out. You might need to try opening a trade for both to work out what ratio you need.
It’s not even that simple, because although they are correlated ie the graphs look very similar, they don’t move together in the same ratio. Market sentiment drives them. For instance if you’d bought silver and sold gold in 2019/2020 because they diverged, you’d have had more than 6000 pips drawdown because gold went very far away from silver. That did of course come back eventually and is an extreme case, but I can see it over 3000 pips a few times. Probably a better strategy in a ranging market once everything has settled down