[Wavytone]

With all lengths in mm here are the relations:

Focal length of the scope = F
Eyepiece focal length = f
Aperture of the scope = A
Maximum pupil of your eye when dark adapted = a, which we will assume is 6mm (in average conditions).

Focal ratio = R = F/A or alternatively F= RA
Magnification = m = F/f = RA/f
Exit pupil of the beam from the eyepiece = e = A/m = f/R

Lowest useful magnification occurs when the exit pupil = the eye pupil, i.e. e=a. Or, when choosing your lowest power eyepiece f = Ra, which for your scope is 30mm. As you have a 25mm Plossl, that's close enough.

For a person with normal vision, the maximum useful magnification occurs between 1.1X to 2X per millimetre of aperture. For refractors and f/15 Maksutovs it's 1.5X to 2X. For SCT's and Newtonians, 1.1X to 1.5X. This is the highest magnification at which you should just be able to discern the diffraction pattern around bright stars. At higher magnifications, objects get dimmer and fuzzier and you won't see any more detail.

So for your refractor m = 1.5A and the eyepiece focal length required is f = R/1.5 or perhaps even R/2, which suggests an eyepiece in the range 3-4mm. The Barlow works as described by Alex. A 3X Barlow and those eyepieces will give effective focal lengths of 8 or 3.3mm. 8 will be fine but 3.3 is very short and probably awkward to use.

You may find a 2X Barlow more useful which, with the 10mm eyepiece is equivalent to 5mm. If you really want the maximum magnification, acquire a 7mm eyepiece which will also give 3.5mm with the 2X Barlow, giving a nice selection of magnifications with just 3 eyepieces.