I see the messy information elsewhere especially in selling infos about antennas for BLE5. OK optimal antenna is a quarter length of average wavelength -(299 792 458) / ( ((2402+2485)/2=2443.5) * 10E+9 ) ) / 4 = 30.67 mm.
But dbm value varies from 1.1 dbm to 11 dbm.
Maybe Ohm resistance of that piece of wire ... sorry usual Whip type antenna with cable from U.FL to SMA-antenna
Maybe Nordic have Guide about it to clear matter ???
Your calculation for a quarter wave monopole antenna assumes speed of light in vacuum, which will not give an accurate estimate when surrounded by air/PCB substrate. See this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_008.pdf?cp=14_19
The length of the antenna is also affected by the size of the ground plane, surroundings, etc. making it hard to calculate accurately. We recommend that you tune the antenna for optimal performance, see this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_017.pdf?cp=14_12
Still, even with a good antenna the received signal power will vary a lot due to multipath and fading.
If you are uncertain about your antenna design I recommend that you submit it here for a free review.