Calibre e-book viewer will appear a blank page between almost every chapter

[Quoth]

Originally Posted by chantsing

it depends on my preferences. 1.8 em is more closed to eassys or Publiced books, For me, i wants a big font which needs biger line-height .

You use the ereader or app settings to set line-height, which fails if set in css. The line-height should never be set in a reflowable ebook.
Also the line-height is automatically more according to actual size of font (font metrics and font size).

[DNSB]

Originally Posted by chantsing

it depends on my preferences. 1.8 em is more closed to eassys or Publiced books, For me, i wants a big font which needs biger line-height .

Not quite true. See attached images where I changed the base font size from 0.75em to 1.0em to 2.0em in the basic body style. The 4th image is 2em with a line height set to 1.8. For my personal uses, the only time I use a line height is when working on a book for visually impaired where I find a line height of 1.5 works better. Basically, line height is a multiplier for the font size so increasing the font size automagically increases the line height unless you try setting the line height in absolute units (32px for instance). Absolute units should never be used in a reflowable ebook.