You have defined variable named "volt" as variable-type float
and then you use this variable "volt" inside the map-function
double voltage = map(volt,0,1023, 0, 2500) + offset;
the function map can only work properly with variables of type integer.
So you should define variable "volt" as integer.
If you don't know anything about variable-types it is about time to learn it now.
best regards Stefan