Raising the precept on the off chance that you might be successful in a pwlb application does not sound like appropriate financial management to me!
Proving that the parish has the ability to repay the loan is not about whether you can raise the precept by an extreme amount - parishes are not capped so in theory can raise the precept by whatever amount they wish. Its about demonstrating that you have appropriate financial management in place, have a parish that is of sufficient size to generate the potential for income generation through the precept to the level required (and not by a stupid amount per household) and demonstrating the general support by residents for the intended project.