Well, yes, you do have to count the number of x divisions and multiply by time/div.
Sure you can make a chart for yourself, just establish the 0% and 100% lines and then draw on some transparent foil the corresponding 20%, 30%, etc. lines.
Then tape the foil over the scope screen.
When measuring on x, adjust the x-pos to make reading easier. For instance, shift the picture to the left until the 10% crossing point is at the start of the first division to the left.