How to calculate Sales Tax using EB Tax on a portion of Invoice amount ?

Hello EB Tax Experts,

We have a requirement, during audit process we learnt that some portion of our product is taxable in one of US state. So, auditors came up with suggestion that sales tax should be calculated on a portion (on some percent) of total invoice. Ex. Invoice is for $1,000 then calculate Sales tax on 25% of Invoice amount ($250). Suppose Sales tax rate is 5% then Sales tax amount should be ($250 * .05) $12.5 in this case. So, instead of going at sub assembly level they came up solution to calculate sales tax on a portion of invoice. Initially