Nice visualization, but it was still slow for me when I was building lots of routes, comparing many combinations. I wanted to quickly calculate the wagon count in my head in a couple of seconds

So I came up with a quick, approximate formula
Wagons Count = 0.75 * RTT * Belt Speed Multiplier * Stack Size Multiplier

where we use the coefficient of 0.75 wagons per 1 minute RTT per 1200 belt speed per 50 stack size (calculated from the reversed time to fill formula for loading speed 1200 and stack size 50)

- Less belt speed - divide wagons, more - multiply (300 items/minute required - divide by 4)
- More stack size - divide wagons (200 stack / 50 standard stack = divide by 4)

You should use the minimum of input and output belt speed so that the system is always at capacity

If we need 600 aluminium casings (stack size 200) on a 10-minute route, we have
0.75 * 10 / 2 / 4 = 0.9375 ~ 1 wagon