No - what G/S do you plan with?
You surely don't just assume still air and then cuff it on observed times at fixes? In the example I gave, what G/S did you use to work out your ETA at the turning point prior to take-off?
And yes, I have done some low level flying in my time. At 420 KIAS, for example, the correction to make good time was based on 'increase KIAS by same numerical value as seconds early or late and maintain for the numerical value of miles per minute'. Thus 10 sec early at fix, fly at 420-10=410 for next 7 min. It worked very well.
But medium level requires proper pre-flight computer work in low speed (<180 KIAS) aircraft as errors are induced to a far greater extent if the wind differs from the planned W/V than in fast aircraft at low level.
Having (presumably) worked out an MDR G/S in your pre-flight planning, what technique do you apply to your track distances to obtain an accurate ETA?