Dynamics GP - Tax Schedule ID percentages - dynamics-gp

Question

Is there a way of getting the total tax percentage of an item's tax schedule ID?
So far all I can guess is that this can be done by getting a sum of the percentages for each tax detail ID at the taxable percentage. For instance, tax schedule ID SP101 has three detail IDs: S 0% OVERSEAS, SP101#0% and SP101#20%.
S 0% OVERSEAS has a taxable percentage of 0% of the item, at a rate of 0%, so can be ignored.
SP101#20% has a taxable percentage of 67.46% of the item, at a rate of 20% and
SP101#0% has a taxable percentage of 32.54% of the item, at a rate of 0%. Resulting in an overall item tax rate of 13.492% (67.46 * 0.2) + (32.54 * 0).
I can probably work this out using the tax schedule master and details tables if required by expanding on the code below, but is there a way that GP does this already (for instance when calculating the tax amount of a line on a sales order) that I can use?
SELECT t1.TAXSCHID, t1.TXSCHDSC, t2.TAXDTLID, t2.TDTAXTAX, t3.TDTABPCT, t3.TXDTLPCT
FROM TX00101 t1
INNER JOIN TX00102 t2 ON t1.TAXSCHID = t2.TAXSCHID
INNER JOIN TX00201 t3 ON t2.TAXDTLID = t3.TAXDTLID
Clarification
Each item is assigned a tax schedule ID in the item maintenance window: in my example, this is the code SP101.
I want to know if there is a simple way of me finding out what tax rate (percentage) this tax schedule ID represents. The SP101 tax schedule consists of three separate tax detail IDs, which each have their own tax rates and taxable percentages.
S 0% OVERSEAS has a taxable percentage of 0%, and a tax rate of 0%. So 0% of the item is taxed at a 0% rate.
SP101#0% has a taxable percentage of 32.54%, and a tax rate of 0%. So 32.54% of the item is taxed at a 0% rate.
SP101#20% has a taxable percentage of 67.46%, and a tax rate of 20%. So 67.46% of the item is taxed at a 20% rate.
Calculating the overall percentage of the tax schedule ID would just be a sum of each of the taxable percentages at their tax rates: Tr = (0 * 0) + (32.54 * 0) + (67.46 * 0.2) = 13.492%
The master tax schedule ID table TX00101 contains just the description and ID without the overall tax rate for the item. Is there a table or view within Dynamics GP that will allow me to find the tax rate of a given tax schedule ID, without having to perform the above calculation to work it out manually?