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Reading test kits

>I have problems reading the PO4 Salifert kit. Basically, you are supposed to
>sit the results down on a part of the colour chart, look down the vial &
>read the results - sounds easy ?
>When I look at the vial with it very close to the chart, I get a reading
>circa 0.1. When I sit the vial on a white part of the chart, I get a
>slightly higher reading circa 0.2. The instructions say to place the open
>vial on top of the colour chart which is white & compare the results looking
>from the top.

Do you really think that in absolute terms it matters whether or not you
can read the difference between .1 ppm and .2 ppm in your test kit?  Do you
have an algae problem?  If not, whatever color you are getting is fine, you
just want to watch that it doesn't get a whole lot higher.  If you _have_
an algae problem, again, the absolute value isn't a problem, you know that
you need to drive it lower. (assuming that you are pretty sure that other
paramaters are in control and that PO4 is a good guess as the cause of your
algae problems)