Corporate tax is not on your earnings like flat VAT. It is on your taxable profit. Every tax-deductible expense is decreasing your taxable profit.
Not to mention, by using this inflated tax rate, you are essentially decreasing your income on P&L which is not correct from accounting point of view.
In future, new module could be implemented which could handle it by estimating your corporate tax and making automatic entries but for now, corporate tax should be entered as an expense once a year by journal entry which is traditional method.