As to figuring out "how many blocks do I need"
The first thing I would do would be to decide on an approximate finished size.
Then go by the "finished" size of the blocks/units I intend to use.
I "think" either with graph paper or by laying blocks out on something - a bed, the floor, or a design wall. (A design wall is preferable, but one does what one can with what one has.)
After I get a layout/overall design I think I like, then I start to do the serious calculating.
Sometimes it's with a ruler or tape measure - sometimes it's "calculating" on graph paper -
Now that I've written this, it seems that I have sort of a "however/whatever" approach.