This again is a judgement call based on what readings you have already had. There is no need to test every hour or every 2 hours every day. Try to catch the nadir (lowest reading in a cycle) and with Vetsulin that could be +3 to +5 post shot and then there may be another smaller dip around +7 to +8. Those lowest points in a cycle are different for each cat and can also differ slightly from day to day. A reading at +2 or +3 will often tell you how fast kitty is dropping and determine how much and how often you need to test through the rest of the cycle but if you can't snag a reading then, I'd definitely try for a +4. Every cat is different so it's really a matter of getting to know your own cat not only from a number perspective but also from a behavioural view. If kitty seems to be off, then a quick test may give you a clue as to why or it may just give you peace of mind. While there are guidelines, much of this dance is unique for each of our furballs.
Elmo is starting to drop so get another test at +3 and see where he's at. If he continuing to drop fast, feed a him a small bit of low carb food (a tsp. or so) and then retest at +3.5 or +4 depending on how much he has dropped to see if he is slowing down.
I have to run out for a short bit but will check on you again when I return. If you should need any immediate assistance (which I doubt) then post in Vetsulin with either a "?" or "911" depending on how urgent the matter is and put a link in Health if it's a 911 so you get help right away. I'm just saying all this in case because I don't expect you are going to have a problem.
