TDS for dummies??

Ok can someone pleeeeaaassseee explain this conversion thing to me in the simplest mathematical terms. Tools:
HM EZ TDS Meter here is some info on it so you fully understand: HM Digital TDS-EZ Meter/Tester Water/ppm/Purity/Filter : Factory Calibrated: our meters are calibrated with a 342 ppm NaCl solution. Meters can be recalibrated with a mini-screwdriver. Fun, translucent blue housing that's very strong and durable. Includes a TDS chart sticker on the back of the meter that explains TDS values. Measurement Range: 0-9990 ppm. From 0-999 ppm, the resolution is in increments of 1ppm. From 1000 to 9990 ppm, the resolution is in increments of 10ppm, indicated by a blinking 'x10' image.
The conversion factor for this pen is 0.5 and fox farms conversion is at .7 how do i add this for example. lets just say i take a ppm reading of 148 before i add nutes to my water how do i calculate?

You add these numbers to your water base line based on how much nutes you use. Use percentage of recommended nutes based on how high you want to take your ppm. http://foxfarmfertilizer.com/hydrofeed.pdf

uhh what????? water baseline?? is this the 148ppm for example? so let me get this right... my water before anything reads 148ppm I add a tiny bit of nutes to my babies and it reads 348 now. Would this mean that I added 200ppms? is it that simple or do I still need to do the x and divide things? If so how would i do the times and divide if Im on a .5 conversion and fox farm is on .7 conversion?

Here's a chart that should help http://www.getbluelab.com/Articles/Bluelab+Measurement+Conversion+Chart.html