Considering Significant Figures in Control Limits Calculation for an X-bar Chart

[#1]

I want to build an X-bar chart for data on the thickness of tablets over 20 intervals (200 measurements). The specification limits are 3.0-3.5 mm. JMP calculated very narrow control limits for me, which are 3.28 and 3.35. In rounding these numbers, the limits become 3.3-3.3. I am interested in having only one decimal place, and I see through JMP's calculations that it does not consider significant figures. What can be done about this issue?

[#2]

The number of decimals displayed in the Limit Summaries can be set by specifying the Format for the column in the data table.  However, it does not seem to change the Label display on the chart