In my experience customization is a lot easier and more organized if you have your own spreadsheet and calculator and know the formula for calculation rather than using TWU's website which while user friendly is difficult to work with in an organized and efficient manner. So I'll tell you the formulas TWU uses and leave it at your discretion whether to use them independently or not:

__Balance:__

The balance is the center of mass, so it's the average of all mass on the frame. The formula you get is:

**r_f=(m_0*r_0+Δm_1*r_1+Δm_2*r_2+...)/m_f**

Where m_0, r_0 are the original mass and balance point, and each delta term represents an added mass with its corresponding location. m_f represents the final mass and r_f is the final balance point. Make sure to keep units consistent.

__Swingweight:__

The swingweight, as we can see, is in units of kg*cm^2. This ties in with the formula we'll use as you'll see in a sec:

**SW_f=SW_0+Δm_1*(r_1-10)^2+Δm_2*(r_2-10)^2+ ...**

Where like previously SW_0 is the initial SW and SW_f is the final one. The deltas are the same as the above. We subtract each position by 10 is because the SW axis is 10 cm up the handle which is usually where your hand will reach. **The units here are crucial!** Mass is to be entered in kilograms and distance is to be entered in centimeters, since the units are **kg*cm^2**. The reason it's so crucial here is because this is a nonlinear calculation (because you're squaring things, not just multiplying and adding) so it's a little tricky to convert units and you can get really scrambled and get unreasonable results. As an added note, you should usually neglect any mass you add in the handle in this calculation, unless you're asking more than about 10-20 g in there.

__Twistweight:__

The TW is similar to the SW except it's on a different axis in a different direction. Up until now, our "r" has stood for distance lengthwise from the butt of the racquet (I hope that was clear btw). When calculating TW however, our "r" will be the widthwise distance from the centerline of the racquet. Other than that it's basically the same as SW and here it is:

**TW_f=TW_0+Δm_1*(r_1)^2+Δm_2*(r_2)^2+...**

This time there's no need to subtract 10 because the axis is at the center line of the racquet, which is considered our "0 point". The indices and symbols are the same as above. I'll remind again, it is crucial to get the units right or you'll get a result which is very terribly off.

I hope I've been of help, I hope I haven't derailed, and most importantly I hope I've convinced enough people to take the calculations into their own hands. Good luck!