If you have your lactate threshold test results, you should have been given your lactate threshold heart rate value. Often you would be given that number and a rough estimate of your other zone values in bpm. If you didn't get the zone numbers, but have the lactate threshold heart rate, you can set the lower end of zone 4 to that value and then estimate the rest of your zones around that figure.
Garmin Connect used to have a simple way to add/remove zones, but they removed it sometime after Feb 5. I created this page which can be used to edit/add/remove zones as you see fit.
That way zones are not as precise. I would like to see heart rate in % of LT, with a things as they are now easiest way would be to put a threshold as a HRmax, but that way ride summary would be totally off.
