Cargo hold status light... Green for space left, yellow for full, red for shot full of holes
Mind you, my favourite idea so far is a variant of TGHC's - showing the empty tonnage left. The reason I like the idea of having a number is so I can think, "OK - 10t left, scoop everything", or "Oops - 1t left, better just grab that cargo container and hope for platinum".
For getting used to it, choose a sun with *big* corona.
Rule number one: you want to spend as little time as possible near the surface, so: fly into it at jumpspeed, when it says 'masslock' use injectors, untill your scoops start scooping. Then get the hell out of there, again using injectors, then jump. Or pre-set a system and hyperspace out (my preferred way, even with hard suns (with very small corona) that works, if you get your timing right.
I used to feel it was too much of a hassle, but now I do it virtually every time: instead of buying fuel, I exit the station, go to sun, fill 'er up, hyperspace out.
w/o fuel injectors, it's almost every time an assured suicide mission, mind you (can be done if you use the hyperspace trick) with fuel injectors, it's a piece of cake, after awhile.