Ok, I happen to have both types of thermometers. The glass tube one, which does not have a mark, and the metal one which has a spacer at the bottom so that you don't accidentally touch the bottom of the pan. That one looks like yours. Except mine is for deep frying - not a candy thermometer.
This metal framed one at the top goes up to 450 and 500 F. What is the top number on yours? This one is specifically for deep frying. (I'm not saying it can't be used for candy but it was not particularly made for that)
and yes
pepper, mine is Taylor No 5917 -
My Glass one has a temperature guide on the insert that shows the guide for both candy and fat. A much newer one from Taiwan. Even though it says it is dishwasher safe - I'm guessing it is
not "drop on the floor and smash safe" But there is not name/manufacturer on it. Because I remember the wooden ball on the metal clip and this one clearly never had one.
So all this to say - although thermometers may be interchangeable between candy and fat - try to ascertain that yours is for candy. If for no other reason than the markings will be easier for you to see and recognize as for candy.
As to your question on depth:-
Pretty sure yours will have a clip on the back to attach to the edge of the pot. Use it, and at least make sure the liquid is at least to your line. If it doesn't reach the line at all then I suggest you are using a pot way too big for the recipe. A
little above won't change anything.
Don't complicate things by over thinking it! You know that old adage... KISS.