Hi, so most of you know my brain works/functions/tries differently than most (about twice a year that’s a good think, the rest of the time, not so much  ) So when I was researching what I thought I would need when I was setting up my shop (cause that’s when I had some money from our inheritance) I was on several forums trying to learn about this sawdust making thing/hobby. On the Router Forum’s there was a guy who was moving into a retirement home, and couldn’t bring his stuff  . He was selling a Leigh FMT Pro, for about 1/2 of what a new one would go for. Plus he had a bunch of the accessories,and made a beautiful wood container for all the pieces. i was about to get my tax refund check, and not all of it was already spoken for, so I bought it from him. So point of my rambling, the Missus would like a spool holder for her threads for the mid-arm machine. I’m thinking a simple ladder design, 6 rungs, 4 dowels per rung. Already got the spot to mount it. ( In other words it has to fit  ). I’m thinking that mortise and tenon would be a nice way to join this, and it’s about time I played with the FMT Pro. So MAIN point here, how does one choose the mortise/tenon size? Is there a formula? I’m thinking 3/4 stock , 1/4 wide tenons? After watching a bunch of the videos on Leigh’s site, I’m a little worried this could get addicting. And advice on this or the FMT is very welcome. I have read the users guide, and watched the videos, but all they do is show one all the different sizes that can be done, nothing as far as a making a decision on what size is best.Thank you, Artie  

Here is a good resource from a very knowledgeable guy:

 

 

 

Thank You, that works!

You know we'll expect pictures Artie as evidence.

With 3/4" boards I would cut a 1/4" tenon for a 1/4" mortise. 

I always do 1/3's for thickness, which is what @John Morris said for 3/4" thick stock. For width, it kind of depends on the overall all width of the stock, for me.