Cannot enter custom grid

Hi,

I'm trying to enter the numbers for the local grid but BaseCamp doesn't allow to enter certain values.
I need to enter:
W055º48.000'
+1
+500000
+4002288.2

This las number is always turned to 1000000.00
How can I get Basecamp to keep the value I enter?

I have the very same values in my 60CSx and works perfectly.

Thanks