Look at this datasheet for the 15HP22:
https://frank.pocnet.net/sheets/084/1/15HP22.pdf
CBS ran them at 20 kV ultor voltage.
Look at this datasheet for the 15GP22:
https://frank.pocnet.net/sheets/093/1/15GP22.pdf
Every manufacturer ran them at 20 kV (19.5 kV in the case of RCA, which is a negligible difference).
Look at this datasheet for the 10FP4 CRT. Most 10 inch sets ran them right at 10 kV or damn close to it.
https://frank.pocnet.net/sheets/201/1/10FP4.pdf
Do you see the pattern. Running at design center maximum is perfectly fine, and in the case of a CRT, often desirable.