I have been added my LME as a late addition and have noticed that that there is a temperature drop to about 180 and it takes about 10 minutes to get back up to boil. Do I add this time onto the the 60 minute boil (so 70 minutes) or just keep as is?
I'd leave it be as there is virtually no difference between a 60 minute boil and 70 minute boil as far as hops are concerned. However, there may be a slight consideration for later hop additions that you may want to get the system back to a boil before adding them. ie, if you are doing something like 60-30-5 hop additions put in the LME with 15 minutes left so it will be back to a full boil by the time you hit it with the 5 minute additions.
I always add the extra time on. I think it is good for the wort, and I like to make sure I get everything out of the hops. What's an extra 10 minutes anyway.
As Bensiff noted, it is best to get the extract in and back to boiling before adding any finishing hops. I usually add my extract at 25 (to go), bring it back to the boil. adjust my times, then put in the Immersion Cooler at 20, and then run with the finishing hops per the recipe.
You will be fine either way. There will be little difference between 60 and 70 minute boils. If you move into all grain you will find that many recipes ask call for 90 minute boils. There are a lot of benefits to boiling longer.
Another option is to add your malt to some boiling water in a seperate pot and bring to boil for 5 or 10 mins and then cool. An additional advantage of doing this is you have 2 smaller volumes to cool so you can cool it faster.
^this. Works like a charm, no trying to reboil, no worries about proper hop utilization, no fear of carmelization. Jamil and Palmer talk about this in Brewing Classic Styles, seems to be the way to add LME late, IMO.