Yes it seems you are correct. I wonder how many corals I have lost with the PO4 swings due to the 714 reading zero?! My reaction is always to over feed after.
Yes once you hit zero or very low on the 714 you really don't know where you're at. I would normally get a difference of about .04-.05 which is the accuracy swing of the 714.
The 736 reads up to .60 so no need for the 714 as if you're that high or above you'll need to do some correcting down.