A) The "Day Rate" should be based on salary, as noted in the question. If you as co-owner don't hold a work contract and thus not have a formal salary, you should use base the day rate on a salary that is comparable for the kind of activity you will perform in the experiment. In later reporting (should your experiment be funded), you will be asked to validate your costs.
B) No, there are no max/min daily rates, but very high rates lead to less effort possible within the budget, and will affect the evaluation of the feasibility to carry out an experiment.
BR, /Jan-Olov, OrganiCity team