In response to the original post....
Calculating the length of a dipole from the formula is the first stage and gives you an approximate length. It is best to cut the dipole longer than the calculated length and then trim a bit off each end until you get it to resonance.
As others have mentioned, the antenna will be affected by it's suroundings and also any bending out of shape that may be done to get it to fit into the space available.
Mounting the dipole as an inverted V will also affect the resonant length and the feed point impedance. The height of the antenna above ground also affects these things.
So you have it up as a inverted V fan dipole which appears to be cut too short to resonate where you want it. Pity as it is easier to cut bits off than to add bits to lengthen them.
You need to add wire, allow extra so you can trim it back, you don't want to keep soldering extra bits on. Do this with both the 40 and 20 metre sections ( I think that's what you originally cut it for). As it's a fan dipole start with the longest, 40M, part and slowly trim each end back until your NanoVNA shows you have it resonant where you want it to be.
Once you have the longest section trimmed then do the same with the shorter 20 metre section. There will be some interaction between the 2 fan dipoles and you should trim the longest one and only then trim the shorter one. Then check where the resonance is on both bands again.... you may need some more slight trimming due to their interaction but
chances are it may be O.K. and the job's done.
If you check the SWR it should be below 2:1, it will probably be lower but it would be O.K at that level. 1:1 SWR is very nice but anything below 2:1 will get you on the air.