Just because it's an 8 gauge it doesn't mean you have to put 1/8 pound of shot down the bore. I have a single barrel 8 gauge and it throws beautiful patterns shooting 1-1/8 ounce of shot with about 90 grains FFg. Yes, that's a "12 gauge" load but like I said, you don't have to put 2 ounces of shot down the bore.
But if you just have to have the full 8 gauge experience, I would go with 2 ounces of shot and an equal volume of FFg. My brother insisted on the full 8 gauge experience and he cut his lip as the recoil pushed his thumb into his face. That was enough for him.
The proper amount of shot is more determined by the weight of the gun than the bore. I like to use a 1:100 ratio as a rule of thumb for the maximum load that can be shot comfortably in a trap shooting situation, i.e, shooting 100 times in a row or all day.
The 1:100 rule of thumb means if the gun weighs 100 ounces, you can comfortably shoot 1 ounce of shot. If the gun weighs 200 ounces, you can comfortably shoot 2 ounces of shot. 200 ounces is 12.5 pounds. (5.7kg for the people in Sweden).
Since my 8 gauge weighs around 7.5 pounds, (120 ounces), it's best shot with 1.2 ounce loads.
For hunting, where you mostly carry the gun and only shoot a few times, you can load heavier than 1/100 the weight of the gun.