You ask how to know if a business expense is a write off or not?
The starting point with the IRS
A. Is the Expense Ordinary?
An expense is ordinary if it is common and accepted in your trade or business.
Example: If you run a web design business, the cost of software subscriptions (like Turbo Tax) is ordinary because most business use tax software.
B. Is the Expense Necessary?
An expense is necessary if it is helpful and appropriate for your trade or business.
It does not have to be indispensable or absolutely required; it just has to be beneficial to your business.
Example: A marketing campaign is necessary because it is helpful in generating customers, even though you could technically run your business without one.
To invest in the business or contribute to a retirement account? Short answer; contributing to a retirement account could possibly provide a deduction while investing in the business as a capital contribution is not tax deductible but the use of the funds could possibly create a business deduction.
Here is an in depth articles that you might find very informative and helpful with your questions about is it deductible for a business.
https://turbotax.intuit.com/tax-tips/small-business-taxes/taking-business-tax-deductions/L5RueYPVS?p...
Here is a number of calculators that could assist you in your calculations
https://turbotax.intuit.com/tax-tools/
