The numbers you came up with are correct. Alternatively, the short one can be called the Multi-tank 2, and the 240mm one the Multi-tank 5. The number refers to how many 2502 reels would fit when holding 35mm film. For roll film I use the 1500 series tanks due to economy. The Multi-tank 2 with the 2502 reel needs 330mL of liquid to process two rolls of 120 film. On the other hand, the 1520 tank with the smaller reel only needs 240mL for two rolls of 120. You will definitely need a set of extension arms for the 1500 and 3000 series tanks.