One advantage of using the online declination calculator is that you avoid the correction calculation for what year the map was made.
The problem is not so much that you might make a math mistake as it is the world isn't so easily predictable.
They marked the declination in the year the map was made and then marked an estimate of how much it was changing each year.
The estimated rates of change are not always accurate. The calculator uses newer measurements of the yearly change as the magnetic pole wanders about.

I suggest taking any maps you might be using and get the correct current declination for them. Then you can mark a short straight line on your map at the corrected magnetic bearing and mark it.
I draw this line in red with the magnetic declination and year beside it.