Half Life    Steady State     Days to Reach
in Days      Concentration    (to 4 places)
===========  =============    =============
     1            2.000            17
     2            3.4142           33
     3            4.8473           49
     4            6.2852           68
     5            7.7250           85
     6            9.1658          107
    etc.

A1: Half-Life in Days [entered number]
B1: =0.5^(1/A1)       [Amount retained after 1 day]

A2: 1                 [Dosage amount]
B2: =A2               [Amount after dose]
C2: =B2*B$1           [Amount after 1 day just before next dose]

A3: 1                 [Dosage amount]
B3: =A3+C2            [Dosage + Previous day retained amount]
C3: =B3*B$1           [Amount after 1 day just before next dose]

Rows 4 onwards are copies of row 3 (cell references adjusted, of course)