I am building a MB-664-1 (IM) with the 3k-ips3.75-2.0 soft touch grips. How do the pros know how to make the distance between the split grip?

The nice thing about split grips is you make the length of the handle any length you want. Handle length is a personal choice. Some people like long grips and others like short grips, your choice. Most of the grips I make for a 6’ 6” rod are from 8” to 10” from butt end of the blank to the butt end of the reel seat. However, I’ve made some longer and some shorter. I do like the NFC soft touch CF grips very much. They look great and are bullet proof.
Norm

If you have another rod with a split grip length that you like, just copy that

Doesn't have to be a split grip rod, any similar rod you have around, or access to that has the right grip length is all the info you need.

I measure from wrist to elbow. I personally like the butt to be right at the first crease of the inside of my elbow on most bass and light freshwater rods. This allows me to work a bait without interference on clothes or body. Fighting fish it is long enough to hold under my forearm for leverage.

Long casting rods like deep cranking rods in bass fishing I make longer because the way I fish them the long handle doesn't interfere with anything when I am retrieving. The long handle aids in casting.

A.P.

I agree with all of the above, and I have made my split grips at various lengths to fit my comfort and purpose.

For example: I have a Med. Heavy rod that is 6'3" used when crawling into brush piles/laydowns, or fishing on a dock with all the poles, rails, and roof around me. My handle is 9-inches long. Easier to maneuver for me.
My largest swimbait rods have 14-inch handles for two-handed casting, and an arm tuck for better leverage on the bait (or larger fish).

Also, once you get the length you want, make sure your split-grip parts fit your hands the way you want. I found that for a spinning reel grip, I like a little longer mid grip because of the way I hold and cast the reel. My baitcasting set ups don't need as long of a mid grip piece (usually around 3.5 to 4 inches is plenty for me). The butt grip piece, for me, feels better if it is around 2 to 2.5 inches long; shorter than that and it sort of rubs my forearm in a way I don't like - feels 'nubby'.

Just a couple extra thoughts to consider.