daal wrote:
I am essentially looking for some way of giving a ballpark number to each of a,b and c so I can compare and plonk a stone down at the biggest ballpark number.
If I understand what you are asking, it sounds impossible. You don't want to do calculations but want to have numbers.
If I try to think what is best one can do without calculations, I would say:
1) Just guess what is sente by your feeling of whether the opponent will answer, and whether you can play elsewhere after that. Maybe you go wrong sometimes, but then maybe your judgement improves with experience.
2) Try to play sente moves at the last moment before it seems, by your guess, that your opponent should play their reverse sente move to prevent it. Better to err on the side of playing too early. Maybe you only lose a ko threat that way.
3) If the area looks like global double sente, i.e. that both players can play there in sente, consider the value infinite and play it immediately. (However, be suspicious about double sente identification and consider that maybe one or both players' moves are not sente...

)
4) With gote moves, just try estimate visually what is the difference between you playing there and opponent playing there. Play the biggest moves first, based on where the difference looks the largest. If your gote move looks like reverse sente (i.e. looks like your opponents move there would be sente), assume that they are 2 times bigger than if it is double gote. Though I'm not sure how to multiply by 2 without calculation

Maybe that can be done as some visual imagination too?
[Edit: tried to make the list more comprehensive and made it more complicated...]
But if you want to have numbers, I cannot see any other way than visualising the positions after your or the opponents first move, and then counting how many points more or less each player gets.
You said you will not have time to calculate during a game. I guess the usual thing is to calculate examples outside your games, and by that train your intuition to better and better guess the values quickly during a game. It's like with tsumego. The more you do them the better you get at guessing how to kill or live in a fast game.