The following formula is involved in the calculation of optimal levels of cash (Baumol's model):

Optimal cash level (Baumol's model):
where F is the fixed cost of a transaction, T is the total cash needed for the time period, and r is the market rate of return.

The following references can be used to cite this optimal cash level calculator (baumol's model):
 Soper, D.S. (2017) "Optimal Cash Level Calculator (Baumol's Model) (Online Software)", http://www.danielsoper.com/fincalc. 
 Lee, A.C., Lee, J.C., and Lee, C.F. (2009). "Financial Analysis, Planning and Forecasting: Theory and Application", London: World Scientific Publishing. 