If you are looking to record progress I’d do it straight out horizontal like you do your erection.
That way the only variation will be how much you are turtled at the time and that will improve for you as you go.
If you measure straight down as it hangs then the variation will be where you measure to and from. Particularly hard deciding where it starts from your body. Also leaning over to look at the measurement will be another area for inconsistency.
But there is no right and wrong when it comes to measuring flaccid and it’s better to have any record than nothing.
I often hear people on here saying they regret not taking enough measurements when they first started. In time you will be grateful for measurements now because these 4 weeks will seem insignificant against a life long practice.