I've been using this technique to multiply numbers with .5 or .25 decimals for years
for example 12.5
12 square = 144 and 13 square = 169
169 - 144 = 25
25 / 2 = 12.5
12.5 - 0.25 = 12.25
144 + 12.25 = 156.25
or 12.25
25 / 4 = 6.25
6.25 - 0.1875 = 6.0625
144 + 6.0625 = 150.0625

Is there a name for this technique or did I unknowingly invent something?
t. not a maths major