No, its not at all impossible to project. Take your last tax return. Look at AGI (adjusted gross income). From that subtract either your standard deduction per your tax return OR the total of your mortgage interest and charitable contributions, assuming conservatively that this is all you’ll be able to take. Subtract your personal exemptions. That will give you taxable income. Multiply by rates shown in the plan. Keep in mind that if you are married, first 50K is at 0%, next 50 is at 10%, next 200K is at 20% and so on.
Compare that amount to what you paid. You will either WIN or LOSE.
Actually it is impossible, because there is no specific list of which deductions, exemptions or credits will be kept and which will not. Without that information, you cannot judge with any accuracy the impact.
You cannot start with your AGI, as your AGI has already had your deductions and exemptions taken out - to do so again would result in double-counting them. You would need to start with your gross income. And you are assuming that personal exemptions would still apply, and that they would be for equal amounts - nothing in the plan Trump posted addresses that, so we do not know.
Will Social Security benefits be taxed? i.e., will they be figured into Adjusted Gross Income? Currently some may or not be taxed.