Total Tricks Shortcut

Please forgive me if the following is well known and/or obvious... I have only just thought of it for myself.

A lot often rides on high level decisions on whether to double the opponents or bid one more. The "Law of Total Tricks" is a guide, and here is a short cut:

First assess (A) the actual total tricks available, then count (B) how many total tricks are required for their contract and for your proposed contract. Then subtract A from B. This is the number of tricks each side will fail by when the other side's contract is making. Then look at the vulnerability and decide what to do.

For example: the opponents have bid and supported themselves to 5. We probably have a 10 card Club fit. My partner has 4 small Hearts, so the opponents have a 9 card fit at best. A = 19. They have bid 5, my partner is thinking of bidding 6♣.  B=23.  So their 5 will fail by 4 tricks if our 6♣ is making: conversely our 6♣ will fail by 4 tricks if their 5 is making.  They were vulnerable, we were not.  She bid 6♣... and our top became a bottom.  But I discovered something.