To understand how to calculate your descent rate under a parachute, watch this talk from the 2015 UKHAS conference.
This chart allow you to choose the best size parachute or streamer for your payload - it is just a graphical representation of standard parachute drag formula.
There are two sets of data on the chart - the dotted diagonal line at the top is for sizing Streamers and the group of four diagonal lines in the centre are for sizing parachutes. Both axis of the chart have a log scale - this takes a bit of getting used to but allows the data for a large range of parachute sizes and payload weights to be held on a single graph. To use the chart simply find the weight of your payload along the bottom. Locate the point this weight intersects with the diagonal lines in the centre and read off the parachute or streamer size on the left.
For streamers the value on the left represents the streamer length - the data assumes that the streamers will have a 10:1 length to width ratio (i.e. the width will be a 1/10th of this value).
For parachutes the value represents the parachute diameter. Of the group of four lines the two inner solid lines represent what is considered the normal range of descent speeds for an average parachute (a parachute with a Cd value of about 1). The upper of these represents a descent speed of 3.5m/sec (approx. 11.5ft/sec), the lower a descent speed of 4.5m/sec (approx. 15ft/sec). These two rates are normally considered to be the upper and lower bound of desirable descent speed - your experience may vary.
The upper dashed line in this group represents the slower descent rate for parachutes with a low coefficient of drag (a Cd of 0.75). An example of this type of chutes are those made from a single sheet of material (so called para-sheets) and are thus less efficient. The lower dashed line represents the faster decent rate for parachutes with a high coefficient of drag (a Cd of 1.5). An example of this type of chute are those made from many specially shaped segments of material sewn together. The PML “DuraChute” sizes fall approximately in this area of the chart.