I am unable to find reliable information on the tuner's actual current draw.
The Split Output Y cable can be used from any output but you must mind the current limit for the output used. As long as the combined current draw of the two devices is less than the maximum available from the output used then all should be fine.
As for the accuracy of the current draw... I guess that would be zero, since I couldn't really find any reference to the current draw... The amount of "padding" required would be directly related to the accuracy in this case
Starving a device for current would be much like letting the battery run low... it better not damage the pedal! Likely it just would not sound good, or, as is the case of some digital pedals, it would either prevent initial power up or you could experience intermittent shut downs.
I think the best bet here would be to experiment with what pedals you can combine together to find the pairings that play nicely together. What other pedals are you using? Are you using outputs 5 and 6 for something that requires the higher current?