"...a bottom dollar 870 Express..." Uses the same barrels as a top dollar Wingmaster.
"...have to count all those little holes..." Don't bother. The density, with no holes in the pattern and where the centre of the pattern hits in relation to the sights at a given distance, is what matters. The size of the shot doesn't make much difference. Mind you, 7.5's to 9's are usually skeet/trap and upland bird loads.
Have a look at these. http://www.opticsplanet.net/picture-...g-targets.html