When I cut blanks for grips, I cut them 2 3/8" wide by 4 1/2" long by at least 1/2" thick. This will make any sized Ruger grip. Of course you need two blanks.
On really expensive wood, I can get by with 2 1/8" in width. The size above just gives you a little to play with. You want to have at least 1/2" in width. This is after the back side has been sanded flat.
I don't buy Mahogany for grips. As Chuck said above it is just straight grained wood. People have to realize that a piece of wood that might make a beautiful jewel box lid will lose a lot of that beauty when cut into something small as a gun grip.
I had a guy send me a piece of flooring (scrap from his home installation). I don't even remember what it was. He mentioned that it was very hard and wanted grips from it. Well, I made the grips and to me they just looked like brown wood grips. Very plain Jane.
Here is what I tell all customers who want to use their own wood or even people that want to make their own grips.
Take a 3 by 5 index card and trace out one of your grip panels on it. Now carefully cut out the part that is the grip. This will leave you with a grip shaped hole in the card. You can place this card anywhere on the wood to pretty much see what a grip will look like. But before you do that get yourself a plastic spray bottle with some rubbing alcohol in it. You can spray some alcohol on the wood to see the color it will have when finished. Never use water to wet the wood. Although it will do the same thing, it will soak into the wood. You want your wood to be about 6 to 8% moisture content, so you don't want to add moisture to it. The alcohol will evaporate before soaking into the wood.
Most people who decide to make their own grips do not have all the equipment that I have so they put a lot of hand work into them. It's a shame to put all that work into a set of grips and wind up with a plain Jane set of grips. So take my advice and do as I suggested above and good luck.
Remember, take your time. Do not try to rush the job.
Foxtrapper, You posted your pic of the wood while I was writing my post. "Hard as a rock" is not number one when I select wood for a set of grips.