A 7&1/4 circ saw is usually enough depending on the size of your timber. You draw out the cuts for the tenon on all 4 side of the timber with proper layout technique. Then lay it out again, this time scoring your layout cut lines with a utility knife. Roll the knife to the scrap side. This will leave your shoulder or face nice and square/sharp. With the timber set so the tenon is horizontal, set the depth of your circ saw by holding to the side of the timber and make your depth about a 16th short of your tenon face. Cut the shoulders to this depth staying 1/16 to 1/8 away from your score line. Roll your timber 90 degrees and reset the depth of your circ saw deep as you can but not so much that you will run into your good wood. Now cut along the sides of the tenon staying 1/16th or more away from your final score line. Note: always check your circ saw for the blade being square to the foot before any cuts. Once you've cut as much as you can with the circ saw, you can try whacking the scrap portion with your mallet. If your cuts were close enough, the chunk will break out and then you will chisel and pare to your score lines. Remember not to try to take more than 1/32 with each chisel&mallet pass. Use your combination square to check your shoulder and tenon for square as you pare. If your circ saw cuts were not close enough, use a hand saw to finish your cut, but be careful to keep your saw square and don't overcut into your good wood. You can even just get close and then whack it with your mallet. The 7&1/4 circ saw is an important tool for large chunk material removeal. I was also taught that when using a large 13 inch or 16 inch beam saw, first make cuts on either side of your piece with the 7&1/4 saw, then come back with the beam saw and use one of those kerfs as a guide for the beam saw. This also reduces the amount of material the beam saw has to cut, so it's not straining so much in the cut. Use the larger saw for it's cutting length, not to necessarily cut more material in a pass.
