Edited to remove bad ASCII art and hopefully be clearer...
As a brand new weaver I had the good fortune to take a workshop with Peter Collingwood. He showed us how to centre a warp in the reed without using any math. I have never forgotten it and use it every time I dress the loom.
Say you want a warp 10" wide and your reed is X length.
Lay a measuring tape along the length of the reed (in this case the reed is laying flat because I'm about to rough sley the warp). So lay the measuring tape with zero at the left end of the reed. Pinch the measuring tape at where ever the reed ends. Let's say 36" just as an example.
With your right hand pinching at the 36" mark (the end of the reed) move your left hand to the 10" mark.
Now still holding both points of the tape - 10 and 36 - bring your hands together and align the 10" mark and the 36" mark, effectively folding the tape together.
By moving your left hand to the 10" mark, you have subtracted 10" from the length of the tape, and then folding 10" and 36" together, you have divided the remainder of the length of the reed in half.
Lay this 'half' down onto the reed again and where the tape ends is where you begin to sley the warp.
0-----10-----------------------------36----------- one hand pinching 10, the other pinching 36
When 10 and 36 are pinched together to the bend in the tape is what you use to measure from the end of the reed to where you begin sleying.
In the photo above, the 10/36 is at the left hand side of the photo (my right side) and the loop is in the 'middle' of the reed. This is where I will begin sleying.
No math. Centred.
Thank you, Peter Collingwood. A technique I use every time I dress the loom.