Each method has its merits and bad points. DA is more geared for when you need to convert more than a few items in one problem and it has a built in answer checker if you keep track of the units, but it can be tedious and confusing at times. Ratio is great because it always works and you really don't need to memorize anything, but you have to keep each conversion separate. Formulaic is a shorter version of the ratio method, but you have to remember what needs to be divided or multiplied.
Personally I use whatever works best for the situation and to be honest I do half of the work in my head. It really depends on what you are trying to figure out. For an example,
You have an order to give 10 mcg/kg/min dopamine using IV tubing with a calibration of 60. Patient is 154 lb. Available is 800 mg in 500 ml NS.
Available;800 mg in 500 ml NS
For the formula method the weight must be converted to kg
Available;800,000 mcg in 500 ml NS or 1600 mcg/ml or 1.6mg/ml
weight; 154lb /2.2= 70 kg
To get the order multiply 10 mcg by 70kg to get 700 mcg/min,
Available;800,000 mcg in 500 ml NS or 1600 mcg/ml
weight; 70 kg
to get the ml/min (700/800,000)*500=0.4375 rounded to 0.4 ml/min
If you wanted ml/hr (700*500*60)/800,000=26.25 ml/hr round to 26 ml/hr
If you really want to go old school you could calculate gtt/min, in that case you take the 0.4 ml and multiply by the drip set, which in this case is 60.
It only looks ugly because I am showing all possible products and the steps.
I would assume you are programming a pump to ml/hr and usually iv drugs have a concentration already on them of xmg/ml or xmcg/ml in the case of dopamine. Note that I pulled a real drug concentration and dosage (except you might start at 2-5mcg/kg/min) off the internet for this example. Dopamine is mix 400 mg in 250 ml of NS or 800 mg in 500 ml NS to produce a concentration of 1600 mcg/ml, which is what you get if you divide 800,000 mcg by 500 ml. Knowing the concentration of the drug takes a lot of work out of calculating the flow rate. If you know the dose is 700mcg/min and the concentration is 1600 mcg/ml, if you divide the order by the available you get mil/min, and if you multiply that by 60 you get ml/hr. so, (700/1600)*60= 26.25 ml/hr rounded to 26 ml/hr. If you are going to do a titration of the medication ((xmcg * kg)/concentration)* 60 min=mL/hr may simplify it if you already know the weight in Kg and the concentration. The short version is ((D/Q)*V= ml, where as D is the dose, Q is the amount of medication in the fluid and V is the amount of fluid and ml is the amount to administer. The long version if you already know the weight in Kg but not concentration it would be, (((10mcg/1000)*70)/800)*500)*60=26.25 ml/hr rounded to 26 ml/hr, could be done on a calculator on the fly. and if you only know the weight in lb, you can add that to the calculation too. So the long long version is...
(((10mcg/1000)*154)/2.2/800)*500)*60=26.25 ml/hr rounded to 26 ml/hr Basically what this string is list of conversions. I would not want to try to remember that even if it is calculator friendly.
If this is something you have to calculate often and on the fly, I would figure out the weight and concentration first, write them down and use this version,
((xmcg * kg)/concentration)* 60 min=mL/hr
Then if the dose is titrated from 10 mcg/kg/min to 15mcg/kg/min, you could calculate this faster. ((15*70)/1600)*60=39.37ml/hr or 39 ml/hr