First, you may need some experimentation, I just took your 25mA for channel and assumed 2 channels. If the preamps are not using 25mA the voltage will change, so... Once you get your 50mA for the supply it's just ohms law, 44V at 50mA it's 880Ω. (44/0.05=880)
Here some answers and notes about the details, feel free to stop reading when you please.
About how critical this value is, I'd assume here that the current is constant for this circuit, if I have 300V or 280V feeding it it shouldn't change much at all I guess, that would be the case for most tube circuits. So the error introduced by the resistor is directly proportional to with those 44V. 10% out of the mark for the resistor would mean 4V out for the PS HV. Less than many other sources of error here, that's why I said 1k will be fine. The option with 2x470 is more to use 2W resistors rather than getting a closer value (could be 2x 1k8 in parallel, closer value)
Another note here about component selection and tolerances. It doesn't make much of a difference trying to get closer to the number, the math will throw an arbitrary number, could been 892.9756Ω. So you pick between the ones in the shelf, depending on the situation you may prefer to go to a higher one or a lower one, or the closer one. If they make the jump from 8.2 to 10 in the resistors values is for a reason, E12 in this case, for 10% parts. Let's go and buy 2 resistors, 1k and 820Ω for example. You know it can't be perfect but you'd expect the 1k to be bigger than the 820Ω. So, 820*1.1=909 and 1k/1.1=909.09. As you see here even if you pick the highest 820 resistor and the lowest 1k resistor in a 10% span the 1k will still be higher. This is simplifying quite a bit the situation, You'll never see one that it's just on the border, because it has to meet the specs over a range of conditions. ±5% is fairly typical for a ±10% part at room temp, normal conditions. If I'd choose 1k and 1k2 and do the same there would be a small overlap, but it's fine, and doesn't make any sense to have 3 digit to express something that may be off by a 10%. You don't say this should be 10.2 but could be between 9 and 11.5, it doesn't make any sense. With the time you make those steps automatically, when you are calculating a resistor you'll need to buy and the calculator says 41.41245k you just read 39k. Of course there are cases that need 0.01% resistors and those came in many more different values and flavours but you'll know when you are getting there, let's hope sooner than you imagine!
Good luck and ask as much as you need, that's what this is for.
JS