Sounds about right. To simplify just load until a cartridge partially obscures the loading port in the tubular magazine. No need to count.
As kids, we'd dump a couple 50 rd boxes of shells in our back pocket and load from there. After a while you get pretty good at coming real close to grabbing the right amount. Not hard to grab a couple more or put a few back either though.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-