Artillery like this is unlikely to come with a range-finder as standard. It's an indirect fire weapon, so the procedure would be to survey the battery position onto the map. Range/bearing information would come from forward observers or even air photographs providing a grid reference, and the firing data would be calculated from that using firing tables which would include adjustments for weather, wind, the earth's rotation, etc. In my very limited experience, it takes about 30 minutes to survey a gun position, although only about two to crash the guns in and out of action. Rocket artillery like this, which requires aiming with the whole vehicle and manual adjustment of the firing angle for each rocket, would take considerably longer, and combined with the inherent inaccuracy would be used en-masse (like Katyusha) Thus it would mostly see use in deliberate attacks where the targets are large, fixed and known well in advance, or deliberate defences where they'd be sited on major (brigade sized) kill zones.
Having said all that, to make the vehicle truly useful in the game, it needs at least to have the ability to have the user adjust the range and some means of estimating it. Coincidence rangefinders wouldn't likely be part of the battery's standard kit - they'd be off with the forward observers who actually can see the targets. However the battery would certainly carry binoculars. All those various graticules lyric1 has put up are just various ways of showing a 10 mil grid. 1 mil is 1 metre across at 1000 metres. When adjusting fire, you just count the mils from where the shell went off to where you want the next one landing, and multiply by the range in kilometres to get your left/right adjustment. The range comes from your map and the Mk I eyeball, and you use range-cutting to get on target (adjust by decreasing powers of two, and alternate add/drops to bracket the target). Or if you know the width of your target in advance (like a hanger say), then you ratio it's width on the ground to its width in mils in your binos, and there's your range.
