Hugh, your maths is right though the reasoning is a little off. A Barlow will increase the focal length of a scope by the factor it states, 2X will double it, 3X triple, 5X quintaple it.
Increasing magnification with any scope will result in a dimmer image always. Better quality EPs and barlow's will increase the amount of transmitted light with specialised glass, lens design & quality lens coatings, and give a better overall image. But increasing the aperture of the scope will also result in a brighter image at the same magnification.
There is also a practical limit to how much magnification a given diameter will permit before the image starts to degrade. The rule of thumb being 50X per inch of aperture. So a 4" scope could yield 200X, 8" scope 400X.
BUT! the ultimate dictating factor is the atmosphere! Typical max. being no more than 150X REGARDLESS of your scope! 250X one rare occasions. Anything higher and you should be buying lottery tickets. This though relates to constant steady seeing with nearly no atmospheric thermal distortion.
If you want a larger image, the only way you will achieve this is by 'stacking' multiple images via software that will clean up the image too. A tracking scope isn't mandatory either for this too. There are practical limits, one being just magnifying the image so much for your scope's aperture (and the quality of its components), that the image just becomes too dim, and cheap EPs, such as plossls have an eye lens that is just too small for projection photography. Stacking will also clear up much of the thermal distortion.
Tony, you may be asking too much of your kit for what you want. It's focal length is short, and I wonder about the quality of all its components.
A better quality instrument will be your principle key to better images. If this isn't practical for you right now, try using other photo techniques. You may be asking "Canon" quality images from a Box Brownie.
Edit: Brendan has touched on much the same things as me,
