Planet X Distance Guide
Calculation of Planet X Distance based on Weeks Left and Solar System Points

When data has an inflection point (e.g.  N1 weeks  N2 miles for Px).
Then a 4 point cubic equation interpolator is needed.  The effect
is to have both a convex and a concave bias in the computed interpolated 
data interval. 
Distance to Saturn's Orbit
    INFLECTION DATA
    DIST.    WKS LEFT
   11.71       13         
    7           9.7       
    1.23        7.1
    0.886       6
                  
  COMPUTED  DATE     PX DISTANCE (from our Sun)
  WKS LEFT           BILLION MILES
    13     13-Feb    11.71
    12.5   16-Feb    11.89
    12     20-Feb    11.64
    11.5   23-Feb    11.03
    11     27-Feb    10.15
    10.5   02-Mar     9.04
    10     06-Mar     7.79
     9.5   09-Mar     6.46
     9     13-Mar     5.13
     8.5   16-Mar     3.86
     8.4   17-Mar     3.62    Pluto = 3.67 bil miles
     8     20-Mar     2.72    Neptune = 2.79 bil miles
     7.5   23-Mar     1.79
     7.4   24-Mar     1.63    Uranus = 1.78 bil miles
     7.2   25-Mar     1.35
     7.1   26-Mar     1.23    Red Cross of Px seen in sky
     7     27-Mar     1.12
     6.5   30-Mar     0.8
     6     03-Apr     0.89    Saturn = 0.886 bil miles
Distance During Passage:
    INFLECTION DATA
    DIST.    WKS LEFT
     1.2        7
     0.886      6
    -0.08       0
    -0.886     -6
                  
  COMPUTED  DATE     PX DISTANCE (from our Sun)
  WKS LEFT           BILLION MILES
     7.1   26-Mar     1.23    Red Cross of Px seen visibly during day
     6.9   27-Mar     1.17
     6.7   29-Mar     1.1
     6.5   30-Mar     1.04
     6.3   31-Mar     0.97
     6.1   02-Apr     0.91    Saturn = 0.886 bil miles
     5.9   03-Apr     0.86
     5.7   05-Apr     0.8
     5.5   06-Apr     0.75
     5.3   07-Apr     0.7
     5.1   09-Apr     0.65
     4.9   10-Apr     0.61
     4.7   12-Apr     0.56
     4.5   13-Apr     0.52    Jupiter = 0.484 bil miles
     4.3   14-Apr     0.48
     4.1   16-Apr     0.44
     3.9   17-Apr     0.41
     3.7   19-Apr     0.37
     3.5   20-Apr     0.34
     3.3   21-Apr     0.31
     3.1   23-Apr     0.28
     2.9   24-Apr     0.25
     2.7   26-Apr     0.22
     2.5   27-Apr     0.19
     2.3   28-Apr     0.16    Mars = 0.141 bil miles
     2.1   30-Apr     0.14
     1.9   01-May     0.12
     1.7   03-May     0.09
     1.5   04-May     0.07
     1.3   05-May     0.05
     1.1   07-May     0.03
     0.9   08-May     0.01
     0.7   10-May    -0.01
     0.5   11-May    -0.03
     0.3   12-May    -0.05
     0     15-May    -0.08    Px crosses elliptic
    -0.2   16-May    -0.092   closest approach of Px to earth
    -0.4   17-May    -0.12    (WITHIN 14 MILLION MILES)
    -0.6   19-May    -0.14
    -0.8   20-May    -0.15
    -1     22-May    -0.17
    -1.2   23-May    -0.19
    -1.4   24-May    -0.21
    -1.6   26-May    -0.23
    -1.8   27-May    -0.25
    -2     29-May    -0.27
    -2.2   30-May    -0.29
    -2.4   31-May    -0.31
    -2.6   02-Jun    -0.33
    -2.8   03-Jun    -0.36
    -3     05-Jun    -0.38
    -3.2   06-Jun    -0.41
    -3.4   07-Jun    -0.43
    -3.6   09-Jun    -0.46
    -3.8   10-Jun    -0.49
    -4     12-Jun    -0.51
    -4.2   13-Jun    -0.54
    -4.4   14-Jun    -0.58
    -4.6   16-Jun    -0.61
    -4.8   17-Jun    -0.64
    -5     19-Jun    -0.68
    -5.2   20-Jun    -0.72
    -5.4   21-Jun    -0.76
    -5.6   23-Jun     -0.8   12 weeks have passed for Px 
    -5.8   24-Jun    -0.84   touching Saturn's orbital path twice
    -6     26-Jun    -0.89   the other side of saturn's orbit = .886 bil miles
The Spline Equation Finder for 4 Points

f(x) = a0(x-x1)(x-x2)(x-x3) + a1(x-x0)(x-x2)(x-x3) +a2(x-x0)(x-x1)(x-x3)
            + a3(x-x0)(x-x1)(x-x2)

           f1 = f(x)  x=x1,  f0 = f(x) x=x0,  f2 = f(x)  x=x2
                    &  f3 =  f(x)  x = x3

                    a0 = f0 / { (x0-x1)(x0-x2)(x0-x3) }

                    a1 = f1 / { (x1-x0)(x1-x2)(x1-x3) }

                    a2 = f2 / { (x2-x0)(x2-x1)(x2-x3) }

                    a3 = f3 / { (x3-x0)(x3-x1)(x3-x2) }

y = f(x) = d      x, x0, x1, x2 =  t = time left in weeks
  y0,x0   y1,x1  y2, x2 = matching data pairs, a0 a1 a2 are calculated