Doesn't a "wee dram" eventually equal a fifth.

That was always my impression.
We were complaining (in 1977) about the slow US conversion to metric, which would have made this process less error prone. My foreman overheard us and told us we were way off base, so ...
The change from English or US standard units to MKG was easier in some fields than in others. It's easy enough to change from one to the other in our formulas, but not so easy in manufacture or repair of mechanical things. A hardware dealer must maintain a supply of both metric and US nuts and bolts even if US manufacturers instantly began using metric measurements because old, sometimes ancient, things must be repaired.
Here is the developer listed in the Air Corps manual of 1941 that is obviously D-76:
Water (about 125 F)----------------------96 ounces.
Metol------------------------------------106 grains.
Sodium Sulfite (des.)----------13 ounces, 110 grains.
Hydroquinone---------------------------- 290 grains.
Borax, granular---------------------------116 grains.
Water to make----------------------------- 1 gallon.
That would be a pain in the rectal region to convert to MKG, and who now measures things in ounced and grains nowadays?
Measuring spoons now would be hard to find in anything but MKG units in the US. Check it the next time you're in a store that sells kitchen utensils.
It's pretty much beside the point. If you have a sufficiently precise and accurate means of weighing, use it for your own peace of mind. I won't tell you to use the spoons unless I have pretty good evidence that particular formula will tolerate the probable error.
The dram is to be multiplied by however many minutes it means when you tell your significant other "I'll only be gone a little while."