Gear Ratio math help required


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Hi all, I've finally got what i need to put gears on to my Huasheng Kit, I bought a 3 speed Sturmey archer Hub on ebay and i have a new gearbox (grubee stage ) to replace my noisy JL gearbox, THe problem i have is my math is poor and i need to work out what size sprocket to use on my Sturmey Archer Hub.

What i would like to achieve is a cruising speed in 3rd gear of 28 miles per hour (45 kilometers per hour) ( engine purring not screaming)

I plan on running a chain from the stage 2 gearbox sprocket to the 3 speed hub which is located between the rear wheel and the stage 2 gearbox, the chain will attach to the existing 18 tooth sprocket, next to this sprocket will be another sprocket attached to the hub by the spoke holes on the flange, this sprocket will take a second chain to the rear sprocket (44 tooth).

Here is a sort of diagram to show locations of components

stage 2 g/box 11 teeth:)

Sturmey Hub 18 teeth:)

Rear sprocket 44 teeth:)

How many teeth will i need for the companion sprocket on the sturmey hub, the icon with the ? inside it anyone able to help me with this i would be very greatful forgot to mention engine is Huasheng 4 stroke 49cc. thankyou in advance Adrian. Perth Western Australia.:)
Hey J pilot, that calculator of your's is really good, very easy to use, within 10 minutes i had my answer, i am so glad you posted,:)