Anyway, I'm building a spice rack out of some left over baltic birch and
hardboard and was wondering what spacing is typically used between shelves?

May I suggest a visit to your kitchen wares outlets to see the
"standard" items available and get an idea of the "standard"
offerrings and the measurements of interest to you.

Also, take your digital camera, tape and pencil/pad with you.

Spices used to come in a rather standard suite of containers when I
was a boy. Now, the containers are part of the marketing efforts and
one cannot count on finding the latest spice with integral grinder in
the same size container the pepper came in.

What is true, however, is that the jars are relatively small and
similarly enough in size that they get lost on the shelf behind their
competition for one's attention and palette.

A strair step affair is sold that allows one to elevate the second and
third (and so on) row of spice jars an inch or so above the one in
front. THis helps.

Anothe option combines this approach with a turntable affair that
makes the most of the space and actually improves access to that
little can of Allspice that used to get lost in the rear.

If you have one of those high-end Kitchen stores in the neighborhood a
visit could be worth the gas and time.