The constant block is showing the text that is entered for the parameter, and is not showing the evaluated numeric value. You do not have control over that behavior of the constant block.
To customize things, you could put a mask on top of the constant block.
Then using the Mask's Initialization code, you could change the value coming from the mask to some quantized value that the mask passes down to the underlying constant block. For example:
desiredInverseValueSpacing = 1000;
valForConstBlock = round(valFromMaskDialog * desiredInverseValueSpacing) / desiredInverseValueSpacing;
% where valForConstBlock is the text entered on the underly constant blocks
% for the 'Constant value:" parameter
Using Mask's MaskDisplay (aka Icon) code, you could use text manipulation techniques to get the specific text you prefer.
For example:
fprintf(num2str(valForConstBlock,6))
