Finishing a couple of G-10 fins that I shortened. The chord thickness currently is 10%, forgot to measure them before I started... grinding, I only own talons that I can compare and they are reasonably thin.

So thoughts on what what is an ideal chord ratio, for slalom AND/OR speed sailing? These are 28 and 30cm fins, but any any comments welcome.

Are you sure you mean "chord"?

decrepit uses 8% he's usually spot on , if you copy this picture you mite need to expand it and print it out, find the foil with the foil the same cord width , cut it out and stick it on to a piece of 3ply you will have the base and the center of the fin

Yes and it is chord to thickness ratio.
Sorry keef, most of my fins are 9%,
As I sail a lot in shallow weedy conditions, I figure it's best to use short grunty fins rather than longer slipperier ones.
I have a speed fin that's 8% because I didn't want to make it any shorter.

That image above is the palm shaped at 8% because there's a mm or so of carbon to go on it, bringing it up to 9% when finished.

So depends what conditions you sail in, a 10% ratio in theory will be better at going up wind and have a lower stall speed, but a bit more drag going down wind. But there again, you can run a smaller 10% fin at a greater angle of attack.

It's all swings and roundabouts!

Unless you plan on getting close to 50kts, then cavitation starts to rear it's ugly head. Not only do you need a very even foil profile with the thickest point a fair way back, chord to thickness ratio also needs to drop, possibly down to 7%, Slowy is the expert here, it's certainly not a problem I'm faced with!

10% is fine.

I measured some of my talons they were 10%, maybe a smidge less. Thanks for input, will make some other measurements when I get my hands on some other examples...

does it matter, I have made fins more and less and as long as the foil is ok they work , you mite find the thinner foil mite take a bit more wind to create lift but if your looking for speeds of high 30's ,40's a thinner foil is the go
for some weed fins you can take 20mm from the tip of the fin without altering the foil, altering the foil can cause problems like if you change the foil you change the leading edge ,as the leading edge is the most important part of the foil

another variable is stiffness, thicker fins are stiffer.

Also, if you tend to run big fins, you can get by with thinner profiles.
Tiny fins might need thicker profiles to give you the necessary lift.