Precision decimal coordinates

Personally, I have always had an issue with the way Garmin handles coordinate entry. True, it is more principal than practical, but principals have their place.

BaseCamp and Garmin devices will display coordinates (not store) to approximately the accuracy of the devices. When coordinates are entered in BaseCamp, they will be displayed to this same accuracy, but the values will be rounded, not truncated. In other words, if the rounded values were stored the error would be +-0.05", not +-0.1" for your data entry. This should get you within the error you require.

In reality, all of the digits are (or were) used to convert the displayed format to the stored format (decimal degrees with + for N and E and - for S and W). To verify this, export one of your Waypoints as a gpx file and open it with a text editor like Notepad to get the decimal coordinates that are stored and convert those values back to dms. BaseCamp's text to number conversion isn't perfect for reasons I do not understand, but the error will be quite small.

As for the principal, it is wrong to assume that entered coordinates were obtained at the accuracy of a consumer GPS; to not make it clear to users whether entered values are kept, truncated or rounded; to be unable to verify that data entry was correct if the stored values are as entered.

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...but I can not enter with such precision in Basecamp...

I think you can. What you see on the screen is a simple display value, not the real stored value.

The sample waypoint you mention, when entered exactly as shown (N39° 43 '01.5285 "W8 ° 15' 53.5285") is actually stored as <wpt lat="39.717091284692287" lon="-8.264869004487991">