using retangles
 

how to calculate fabric strips height and length to get a finished size of 20x23

Always add 1/2" for seam allowances on each direction. If you are piecing rectangular units to get that size, add another 1/2" for each unit. so, if you were doing an elongated 9 patch, you would add an addition 1/2".

Hmmm ... the 20" is easy, sure you figured that much out, but the 23 is an icky number.