the problem with combo pens is that if it takes a 'dump' you lose all your meters...I would go with ph and follw directions or use 1/2 strength formula.

you shouldn't have a problem if you follow directions...

the ph, however is critical in my opinion...unless you use ROwater you will fight ph all the time...consistent ph is one of the most important necessities, in growing healthy plants...rememher...ph...too high or too low and your plant will not uptake vital nutrients...So, Imho, get a ph pen! lw