Using map co-ordinates and map regions

I have a problem that I hope someone can help me with.

I have a map with a standard numerical x,y co-ordinate system. Within the map I have regions that are all the same size e.g. co-ordinates 0,0 to 19,19 are Region A; 20,20 to 39,39 are Region B going up to 99,99 in Region E then continuing in the next 'row' of regions such that co-ordinates 0,20 are in Region F; 24,49 are in Region L etc.

I want excel to return the region when a set of co-ordinates are entered. Now for a small map with only a few regions I can use the IF and AND/OR functions.

However my map is now 500 x 500 squares and the regions are 50 x 50 squares (i.e. 100 regions going from Region 1 to 100 in a 10 x 10 array). I can't figure out if there is a relatively simple formula to achieve this.

Any suggestions?

Thanks for the welcome and reply. When the map was smaller I had been working with the INDEX and MATCH functions. As the map grows and the region size changes it means having to regenerate the 2D table which is manually time consuming. My question related to one of 2 potential solution:
First was whether I needed a 2D 'lookup' table at all or whether there was a way of writing a formula that would not rely on a 2D table.
Second was if indeed I do need a 2D table was whether I could use VBA to repopulate that table with data when the map and region sizes change.

However for now I can use the 2D table method. The map will not change appreciably for a few weeks so I'll keep trying.