Let me add a few measurements from the past days in my location - and there's one more week with rain forecast
(I only got my WH40 on 29th, hence a few days with missing data)
Sep WH65 TFA manual WH40
25 3,8 3,3 ---
26 18,0 17,8 ---
27 1,0 1,8 ---
28 2,3 1,8 ---
29 6,6 6,8 ---
30 2,3 2,2 1,8
Oct
1 5,6 5,6 3,6
2 1,6 1,5 1,6
3 6,3 6,0 3,8
So far I observe that the WH65 is usually pretty close to the manual reading (I'm using 2 different manual TFA devices, small one and a larger one [larger=the one Ivano also uses],
but they show the same results so far (+/- 0.1 mm eye reading error)).
The WH40 is systematically underreading (why it was not underreading on the 2nd of October I still have to figure out - maybe the type of rain [granularity/size of the drops] - it might be a runaway value; there's still at least one more week to go).
For me I want to find if I can get a calibration factor, and of course which one.
Of course this will be one pertaining to my location and may not be transferable.