Your deed or title or probate documents may should have a description on it that will help narrow down your net acres. The simplest thing to do is ask the one who offered how many net acres they have for you. If they do not know, that is a clue that they have not done their title work yet.
Here is an example: Deed says an undivided 1/2 interest in the NE4 NW4 of section 11-23S-36E. You actually read the description backwards from right to left. Whole section is 640 acres. The NW4 would be 160 acres and then the NE4 of that would be 40 acres. You have a 1/2 interest, so that would be 20 net mineral acres.
I am attaching an article for Oklahoma which has some good pictures and explains the Rectangular System. Someone may have a similar one for Texas. Note that due to the curvature of the earth, some of the northern tier and the western tier of townships may not have exactly 640 acres. You have to go back to the original patent documents and any newer surveys to see the maps with the plat acreage on them.
S-T-R Legal Land Descriptions in OK_Kletke-1.pdf (59.7 KB)
Here is the link to the patents. Home - BLM GLO Records. I didn’t catch which county you are in, so couldn’t look it up directly for you. Use Search Documents tab