Given a basic X & Y grid what is the formula to determine distance between two grid items?
First item is at grid 5, 6 second grid item is at 7, 8.
No it is not a form of Pythagoras' Theorem.
It is the grid-metric or the 'taxicab' metric.
If $\displaystyle P(x_1,y_1)~\&~Q(x_2,y_2)$ then the grid-metric is $\displaystyle d(P,Q)=\left| {x_1 - x_2 } \right| + \left| {y_1 - y_2 } \right|$