There is a better way. First of all, you should have a preamp before the crossover, in order to change the volume of everything at once. That way, after you get the balance of sound right between the woofer and tweeter, you leave the volume controls on your separate amplifiers alone, and just use the volume control of that preamp that feeds the signal to the electronic crossover, whenever you want to change the volume.
For setting the relative volume of the two speakers, you might want to try a series of test tones and an SPL meter, keeping in mind the fact that different frequencies are going to be different volume due to the fact that no speaker is perfect (e.g., that is, how loud the tweeter will be will vary some according to the frequency fed to it), and also due to room acoustics. Try to get the balance of sound as close as possible at the frequencies closest to the crossover frequency, so that there is not a sudden shift in volume when the sound is coming from one or the other at frequencies that are close together.
Also, keep in mind that if, after playing with it for a couple of hours, you are dissatisfied, you should remember that you can always readjust it later on. And it will never be "perfect", so do not stress yourself too much about it, and just try to get it as close to perfect as you reasonably can.
Although an SPL meter and a test tone disc are not expensive, if you don't want to spend any money at all, you will have to do it by ear. But you can probably find test tones to download for free from the internet (you would have to do a search or ask someone else where that would be), which may or may not be helpful to you. If you just use music, try to find music that plays just above and just below the crossover point, to make sure that that instrument does not sound strange going back and forth from the tweeter to the woofer. Also, double check it with a variety of music, to make sure everything seems okay. The more natural the sound, the better, assuming that you are familiar with how the instruments actually sound in real life. (E.g., a recording of a piano should sound like a piano, and the volume should not shift when a scale is played going between the woofer and the tweeter.)
Of course, you don't have to set it for the flattest frequency response if you prefer the sound to be unnatural, with exaggerated treble or exaggerated bass. You can just use personal preference, if you wish to do so. But I would try the above to get the frequency response as flat as I could.
