My understanding was that the single mark near the bottom of the dipstick is the fill mark. When I drain the transmission and put about 1/4 L of oil in it, that's where it comes up to. (Actually, since I can't fully drain the transmission through the dipstick hole, 250 mL of oil is too much, I usually go for about 200 mL.)
If you clean the dipstick and find that the oil comes halfway up it, I think that means the transmission has been overfilled substantially. I have heard this is Really Bad. The yard does an engine inspection for me every spring, and they made a point one year of pumping out some oil because the transmission was slightly overfilled.
Transmission oil generally does look clear because it doesn't have to contend with the carbon from the engine. That doesn't mean it is clean and that it isn't causing undue wear on your transmission. As a matter of principle, I drain and refill the transmission oil at every haulout. I do it first, before changing the engine oil, so I don't contaminate the oil in the transmission.
My view on this: Oil is cheap. Transmissions are expensive.
