New to this forum.
Question. what is the smallest diameter flat on the nose of flat nose boolit I can use in CB Marlin 45-70 cal. The boolit in question is a Saeco 021A G3B. TIA. Mygila aka My Mangum, Las Cruces, NM

Welcome to the forum. Rules are to join in, have fun, and play nicely with the rest of us kids.

Don't know the bullet, but the simple answer is if the flat is wider than the primer pocket on your cases, it should work. The only object is to prevent the bullet from poking the primer on the next round in the magazine when the gun recoils.

Measure the width of the primer pocket. Not the primer, but the actual pocket.
The bullet nose should slightly more than span this distance, so the nose rests upon the brass on each side of the primer pocket.
I would not use any bullet with a meplat (flat nose) that isn't slightly wider than the primer pocket.
Been shooting the Marlin 1895 since 1977. It's a very fine rifle but avoid the temptation to hot-rod it with maximum and maximum-plus charges. Your rifle will last longer, and your shoulder will too!
Any loads you find posted on the internet should be checked against manufacturer recommendations.
There are many insane and downright dangerous loads for the Marlin .45-70 posted on the internet, chiefly by ignorant relaoders.
As rifles go, your Marlin would be a Group II -- stronger than the Trapdoor and Sharps (Group I), but not as strong as the Ruger No. 1 or 3 (Group III). Some would redefine the Marlin 1895 as a Group II-1/2 but play it safe and stick to Group II loads.
A 400 gr. bullet at 1,650 to 1,800 feet per second (maximum) is not to be sneezed at. Our ancestors shot bison, the big bears, moose and elk with 400 and 500-grain lead bullets at 1,200 to 1,300 fps without a problem.
To hear today's .45-70 hotrodders tell it, they were badly undergunned. Tell that to the bison!
Velocity is no substitute for marksmanship.