schemedraw node distribution not evenly

Question

The elements Cx not distributed evenly, what's proper way to make it distribute evenly?

import schemdraw
from schemdraw.flow import *
import math

pngname = "./demo.png"

def d2r(degree):
    return degree*math.pi/180

with schemdraw.Drawing() as d:
    u = d.unit
    w = u/4
    h = u/4
    dx = d.unit/3
    dy = d.unit/8
   
    d += Box(w=w,h=h).label("A")
    d += Arrow().right(dx)
    
    d += (P := Box(w=w,h=h).label("B"))

    d += Arrow().theta(45).length(dx/math.cos(d2r(45)))
    d += Box(w=w,h=h).label("C1")

    d += Arrow().at(P.E).theta(15).length(dx/math.cos(d2r(15)))
    d += Box(w=w,h=h).label("C2")

    d += Arrow().at(P.E).theta(-15).length(dx/math.cos(d2r(15)))
    d += Box(w=w,h=h).label("C3")

    d += Arrow().at(P.E).theta(-45).length(dx/math.cos(d2r(45)))
    d += Box(w=w,h=h).label("C4")
    
    d.save(pngname, transparent=False,dpi = 150)
print("[[%s]]"%pngname)    

Output: