Ok, so there's likely some calculation kicking around that says X amount of honey, will raise the gravity to Y in Z quantity.
Don't know what it is, but pretty sure I've read something about it, but it presumes standards i.e. the quantity ? fine, but the increase in gravity relies on the % sugars in the honey.
So unless you know exactly the sugar content, its gonna be hit and miss whatever.
Equally, do you know exactly what gravity or sweetness level you want ?
I'm gonna suggest that you just trial and error.
Now, if your batch is already clear, then there's a possibility that the back sweetening honey may cause a haze (its why I usually back sweeten to my taste level - 1.005 to 1.015, after its first or second post ferment racking(s) ). I "think" the haze like this is a protein issue.
It means it only gets cleared once, but the ferment must be finished, and either the yeast dead from exceeding its alcohol tolerance or by stabilising first.
If you check gravity once the batch is confirmed stable as above, measure gravity and taste.
If you wanted, you conduct a sweetening experiment with say a pint ot two adding small amounts of honey mixed with water (say 50/50) and add that. Not only will it show whether there's any possibility of hazing but also how much you'd need to add to the main batch to extrapolate the numbers for total required honey i.e measuring and tasting, tells you what the new gravity is and whether you like that level of sweetness or whether it needs more etc.
doing this a bit at a time can be done with the whole batch to get the taste you want, if you go over a little you can use acid to mask that......
As for boiling/skimming etc? A lot of use who're familiar suspect that that was originally done because any water used wouldn't have been clean/safe and it was just habit that's carried over, only to find that it's not really necessary.
What it does do, is clump any proteins into scum that's skimmed. Equally heating honey like that will drive off a lot of the varietal aromatics and VOC's, rendering the honey much more bland like cheap store bought stuff.
Personally if I get any hazing from sweetening an already cleared batch I just put it back into the jug/carboy and let it drop out naturally.
If you're feeling impatient, 2 part K&C finings usually does the trick.
And no, I don't heat/treat the honey for back sweetening.
So it's up to you really......