Should a suspension with tetracycline HCl and lidocaine be made with lidocaine base, or should we use lidocaine HCl and take pH 5.7 with Na2HPO4 2 aq?

This answer was created with the help of pharmacy Smits-Robrechts van Oud-Turnhout in particular thanks to pharmacist R. Derijnck

I suppose that also lidocaine HCl can be used. If I go out of the tetraycline suspension FNVI then I find that for 500 mg tetracyclineHCl 185 mg Na2HPO4 2aq are needed to set a pH = 5.7. Given 0.5% lidocaineHCl solution has a pH = +/-5 Then another small extra will be needed for the lidocaine. Use of 200 mg showed that we reach a pH = 5.7.  

The delivered composition was as follows:   tetracycline HCl and lidocaine HCl, 500mg each, 1 g hypromellose, 5 ml propylene glycol,   30 ml sugar syrup and water Up to 100ml. I suspect that the shelf life, stored in the refrigerator, should be made at 2 weeks. Maybe even longer but the delivered quantity is limited. In this case, the shelf life can be followed by the colour. After all, at degradation, tetracycline discolours