There is possibly a set ...

There is possibly a set formula to undertake this calculation, but I will use a simple water balance and then the total is either added or subtracted  to the surface elevation height to obtain the answer.   Please note, units of measurement are everything with this problem. 

Starting with rainfall and evaporation; +0.145 m -0.061 m = +0.084 m; inflow and outflow, +6.0 m3/s -6.5 m3/s = -0.5 m3/s;

*Assume that month constitutes 31 days, therefore 31 days x 86400 sec/day = 2678400 sec/month x -0.5 m3/s = -1,339,200 m3/month.  (1,339.2 ML)

*Assume 1 megalitre (1,000,000 L) covers one hectare to 100 mm.  As the surface area of the lake is 5,000 ha, this is 50 ML (5,000 divided by 100 mm) and 1,339.2 ML divided by 50 ML = 26.784 or -2.6784 m

Our water balance is therefore; +0.084 m in and -2.6784 m out.

Take one from the other  = -2.67 out.

The surface datum point at the start of the month was 103.2 less water out of -2.67 equals a height datum at the end of the month of 100.53 metres.

Regards, Kevin.