Prior to the advent of MyRoute-app I have always plotted my route in Tyre then imported it into Basecamp to check that the route is as expected before uploading it to my device. I have just tried experimenting with plotting a route in Tyre and importing it into Basecamp and also into MyRoute-app where I have converted it to Here maps ready to upload to my Garmin. In Basecamp one section of the route is re-routed (it's OK, maybe a better option) but the same section of the route is not shown as re-routed when I convert to Here maps in MyRoute-app. However, when this route is uploaded to my Garmin it reverts to the same route as Basecamp suggests ie not what is shown on the screen in MyRoute-app which does not recalculate when converting to Here - it simply produces the same route on a visually different map. However, if I plot the same route using Here directly in MyRoute-app then I get the same route a Basecamp and it is accurately uploaded to my Garmin.
This means that a route created on Google maps is not accurately converted to Here maps as it is uploaded to the satnav - and I am guessing this may be the case for TomTom maps also - so for a group of friends with different devices the routes may not transfer as expected.
So, what I am saying is ... would it be possible to add a recaculate button when converting from one map to another so that you get an accurate representation of what will be uploaded to the device and extra waypoints can be added as necessary to force the desired route.
Each card has its own calculation, which can not be otherwise.
But you can check the route in different maps and customize.
And the advice is to navigate with the GPX track.