Through (tusk) Tenon help needed

Hi, I am hoping I may get an answer here before I make the plunge on the next step of the bench. After a shop disaster I was able to get back into the shop Saturday.
What I had planned was for my bench base is to have a tusk tenon. However the dimensions I have are kind of worrying me.
My base posts are 3 3/4" thick and 5" wide.
The stretchers are 2 7/8" thick and 6" wide.
So here is my dillema. I want to make a tusk tenon, with a total thickness of 3 3/4" I need to create the shoulder and through piece. In rules of thirds this would mean the shoulder would be 1 1/4". So that being the case I would have to again split this in thirds to determine the tusk size which in turn would be close to about 1/2".
Does this seem right? The top right now weighs aprox. 175+ lbs.
Thanks ever so much!

I haven't used tusk tenons so that minimizes the already low value of this comment. The tusk tenons I am familiar with use only upper and lower shoulders so the width of your stretcher would remain constant:

is there a reason you want to use a wedged tennon over a crossed metal dowel?
the metal will offer a much stronger/more adjustable connection....but it ain`t "cool lookin`"

I am opting against bed bolts or metal fastener's for the base, well for almost the entire bench. The only bolts are the two that will fasten the endcap to the bench holding the wooden screw tailvise. I also want to keep the "breakdown" aspect of the tusk.