The fin measurement I'm familiar with, is the column of water that's disturbed by it's passage thru the water, in other words the tip's right angle distance from the projected board base.

There's no reliable conversion between any weedy and any pointer.
A lot depends on if the weedy is designed from scratch or is a standard pointer blade reboxed at 45deg.

Raking a fin back increases the chord but not the thickness, so the chord to thickness ratio changes, in the case of 45deg the ratio is 20% less.

Then there's flex!!!!!
In a pointer a small amount of flex will give the fin a vertical component to it's lift.
In a weedy any flex decreases lift and adds to drag.

So it all depends on the weedy, there's a lot of duds out there that give weedys a bad name.

Fin depth, affects the leverage of the fin over board angle, to get the same leverage from a weedy as a pointer, the weedy will probably have a bigger area.
so a bit more drag.

On our Estuary we can get away with 35deg in the deep, as the weed on the surface usually isn't very thick. But if we're speed sailing in the shallows 45deg on the bottom half of the fin is essential.
Depends how fast you're going, fins pick up weedy easier the slower you go. Taking off in heavy weed is tricky even with a 45deg rake.

Thanks for the reply Decrep although it only kinda answered my question. I did just stumble upon an old thread that was very very interesting about the mechanics behind weed fins and particularly Sam's and your observations. One thing I took from it is that Select pointer fins make good weedies and it just so happens I have a 32cm Select pointer I don't use anymore. I'd love to turn that into a small weedie for my speed board.
Now all I need is some kind Seabreezer to offer to rebox it for me (Keef) for a fee no doubt
And yes Keef, I am thinking about the Lake and Winter

Happy to post it to you Keef......Maybe I should rebox the BP weedie with a greater rake also...???

G'day Rex, an easy way to remember the ratio is, the length of the pointer you wish to rebox at 45 deg divided by 1.41. So your 32 pointer will make a 22.695cm weedie at 45 deg. Also if you're going to trade off a little drag for lift, 40 deg works out at 1.38. Muf (ultra) use a 40 deg fin with half the fin at 45 deg. Makes sense but I havent tried them yet, for weed just under the surface.

with the wider based weed fins you will get heeps of lift to windward ,but the board tracks flatter, same as off the wind, if i use my 21 slingshot the board feels lively and sailing off the fin, with the weedy you dont get that same feeling , after saying that i can stick a 17.5 weedy on my ca55 with a 4.6 to a 6.2 race sail = 15knts to 30, ok so you loose a bit of topend speed in normal conditions , if you stick them in a speed board with tail rocker in flat glassy weed conditions its a different ball game
ok to put it in a nut shell, your average slalom board is to flat to get the same felling out of a weedy= horses for coarses rexy

cant believe these two fins wernt snaped up for $99 each www.seabreeze.com.au/Classifieds/Windsurfing-Accessories/~a4v15/2013-Maui-Fin-Company-New-Maui-Ultra-Delta-Speedweed-Fins.aspx?search=v7hoBSc7Ja%2bEnNu9O0B1qw%3d%3d&t=0

the g10 these fins are made of is bullet proof, well almost, i will be reboxing them too 18 & 19cm at 47deg, what i have found by reboxing the 15 to a 17 is the foil is moved forward and helps the fin to wind ward and gives the fin more control off the wind ,
i'm not sure if that's the technical term but im investing $200 on my theory

don't get me wrong , carbons great but it doesn't take much to destroy the leading edge, so i'm working on finding a solution or call it a hybrid

theres no point in havening a fin that will do 40knts at the start of the session and then cavitates at 30knts 1/2 way through because the leading edge is stuffed