That is odd - most of the patterns I have looked at tell the exact size of pieces I need which generally includes a 1/4" seam allowance. If the pattern DOES NOT indicate how many strips to cut, then the math is going to be a little more intensive.
I use 42" as the general fabric width for my calculations because it is the shortest fabric width I've come across. Then I divide the length of my piece into 42. Let's say my piece is 4" then I get 10 pieces out of each length. (42 divided by 4 = 10.5)
Then I figure out how may pieces I need total (pieces in each block multiplied by the blocks in the top) and figure out how many strips I will need. Let's say I need 4 pieces in each block and have 30 blocks so I need 120 pieces. Each strip gives me 10 pieces so I need to cut 12 strips.
If each piece is 2.5" wide, I know I need to cut 12 strips @ 2.5" and I need to have a minimum of 30" of fabric for that color and piece.
You would need to repeat that process for each piece in the block.
IMPORTANT: I always assume that I will mis-cut something or get off on an angle, so I always buy more fabric than the minimum. Generally, I need to restraighten my cut line at least once.
IMPORTANT: Depending on the complexity of the block, I make a trial block to test the pattern BEFORE cutting all of the fabric and finding out there is a problem with the pattern.
Just so you know, I usually have to go back to cutting because something was mis-cut, mis-aligned, sewn wrong. To me that is just part of the process. Patchwork is not always a linear process. Sometimes it is darn right wonky, lol
Hope this helped.