To go below the lowest power of my FT-817, I use attenuators of 10 dB and 20 dB. The attenuators are built with resistors of 5%, but the accuracy is far better than 5%.
A few years ago the homebrew attenuators were measured, during a visit to Mischa PA1OKZ. The 10 dB attenuator turned out to be 9.95 dB. The 20 dB attenuators were 19.92 dB and 19.95 dB.
An excellent result for homebrew attenuators. So I have no doubt about the accuracy.
The accuarcy is much better than I expected.
Because I built the attenuators for a power of more than 1 watt, I had to use more than 6 resistors.
I calculated the values of the resistors with a pocket calculator and built the attenuators with resistors that were at hand, using the table from a informative article of Douwe PA0DKO.