After doing starch tests (spraying potassium iodide solution on cut apples) for many years, we now tend to judge ripeness from looking at the background colour, and to a lesser extent taste.
When the background colour goes from pale green to pale greenish yellow, that is a pretty good indication they are just about there. If the background colour is yellow with no sign of a greenish tinge remaining around the calyx, they probably aren't going to get any better being left on the tree.
If it is a variety with no background colour visible because there is so much red pigmentation (eg. Liberty), then it is when the colour goes from dull red to a bright "glowing" red. If there is green under the red, the red is dull. If there is yellow under the red, the red becomes bright.
We have seen many of the same issues. Gravenstein will drop readily here - they seem to ripen so rapidly that they can look green and immature one day, and then 30% might be yellow and on the ground two or three days later.
We don't have too much trouble with Jonagolds dropping. We do thin them, but can keep the size down to some extent by limiting the amount of irrigation the trees receive. We find if we don't thin Jonagold they tend to be paler, less flavour and softer.
We have also found the relative ripening time is not consistent between varieties every year. Age of tree, crop load and seasonal conditions (maybe amount of winter chill and weather conditions in Spring?) can have a bearing on ripening time. We too have found sometimes Elstar is before Gala, and sometimes after.
Probably safest to just keep checking the background colour, and taste.
Kalangadoo OrchardOn the “other side” in South Australia