I know this isn't the answer you're looking for, but honestly, I've been caught out a few times and the only realistic safe option is to measure/calculate each individual engine. There's just too many variables - if the deck of the block has been machined (they can vary a few thou even when brand new - as they might have had to take a slightly bigger cut during the manufacturing process, or when the tooling on the mill was on its last legs, then the same deal for the head, and how new valve seats might end up with enough valve installed height (vs the head deck surface) that could alter it by a cc or two.

Basically you really need to measure it as best you can. A 'trick' if you like - you can usually find a local glazier (people who make windows etc) and find a 4x4 discarded piece they could cut square. Or even better - somene like a small workshop that repairs clocks and signs - might have a small piece of perspex. For the actual measuring, thankfully, the chambers aren't massive, so you can get it done in 2 'stages' with a big enough syringe. In the past I went to a veterinarian, and got a syringe (no idea what it was used for) from them that was marked and held 20cc's. So two goes of that and a measurement is possible, and it'll be quite accurate (whereas if it took 4 or more goes - for some really big chambers with more than 80cc, you'd introduce a small error each re-fill, so it might be out a cc or 2).

As a very rough rule of thumb, you can calc the compression it has now, then work out what chamber volume (the math is easy and there are also online calculators available - let me know I'll post a link) you'll need to get your desired comp ratio. Then sit the head dead level (as best you can) and fill it with the amount of fluid equal to the volume you are chasing. A careful look will be able to pinpoint where it is on the chamber, ignoring the meniscus (curve of the fluid near the edges). Then you can measure or guesstimate how much it needs off it.

Let's say it hypothetically needed about 50 thou off to get the comp ratio right. I'd suggest getting 25 thou off, then re-measuring, and then 10 thou at a time. It's _very_ easy to set a head up to machine the deck on it, so thes extra cuts (providing the machinist is a reasonable bloke) shouldn't have to cost a fortune extra, and its a _hell_ of a lot better to creep up on the comp ratio than to do one cut and go too far.

Obviously for a streeter this next thing would be overkill, but if you were racing and had no other source of a closed chamber head, the 'go' would be to weld up the chambers to make it like the closed chamber head style. Unfortunately that's a lot of welding, and you'd have to pull out all studs, valve guides valve seats etc - make it completely bare, heat slowly all over, weld, then cool slowly (even for alloy) then practically all surfaces need to be machined and trued up. But sometimes that's just the way it has to be.

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but back to reality - if at all possible, you'd try and hope that it could be planed enough to get rid of the extra dish area and make it a closed chamber job. that chamber shape produces more power (even for the same comp ratio) and is more detonation resistant than a big open chamber and no quench. It actually allows you to run more comp for the same fuel (for example with a particular cam and open chamber head if 10.5:1 is safe, then a closed chamber head with tight quench clearance would be safe with perhaps 11:1 or even slightly more.

Having said all that - obviously compression (where it can be safely increased) gives more power, and better fuel efficiency at part throttle (talk about win win), but it is a question of 'how much'. Here's the thing - for a moderate (but bigger than std) street/weekend racer type cam, running 10:1 vs 10.5:1 is probably only going to be a difference of a few percent power/torque (maybe 2-3, certainly less than 5%) It's only where you go from extremes - say 8:1 compression from a short duration cam and made to run on lowest grade pump fuel - going from that to a bigger cam will run ok, but be a little sluggish, and going from 8:1 to 10.5:1 would make the 5% or so difference. Interestingly (or not) whilst it only gives about a 5% gain in power, it'll likely give as much as a 20% improvement in fuel efficiency at part throttle cruise.