There are a few ways to enter waypoints for portages or points of interest ahead of time. I have used the method you came up with of using a metric ruler and then estimating the UTM numbers for the easting and northing position. Since the UTM numbers are actually distance all the way down to meters for the last 3 digits it works OK.

With the advent of the Canada Topo maps on the computer it is now a lot easier. With the park map at your side and the desired lake zoomed in on the computer screen and the waypoint tool enabled it is a simple matter of moving the cursor to the desired spot and clicking. This will record the spot as a waypoint. I advise naming the waypoint to avoid future confusion with the default number that is assigned. Record all these and any other info from other sources into one file for your trip. You can use copy and paste to glean items from others files.

When complete upload the file to your GPS.

A few cautions:
Avoid the default numbers because only one of duplicate numbers will be stored in the GPS.

Garmin only reads the first 9 digits of a name, hence if there is a list that all have the same first 9 digits it will only load one of the list.

Keep separate files for different aspects and trips.